diff --git a/lib/covered/coverage.rb b/lib/covered/coverage.rb
index 2ca3bc6..3b324af 100644
--- a/lib/covered/coverage.rb
+++ b/lib/covered/coverage.rb
@@ -51,6 +51,16 @@ def initialize(source, counts = [], annotations = {})
 			@annotations = annotations
 		end
 		
+		# Initialize a copy of this coverage object.
+		# @parameter other [Covered::Coverage] The coverage object to copy.
+		def initialize_copy(other)
+			super
+			
+			@source = other.source.dup
+			@counts = other.counts.dup
+			@annotations = other.annotations.transform_values(&:dup)
+		end
+		
 		# @attribute [Covered::Source] The covered source metadata.
 		attr_accessor :source
 		
diff --git a/lib/covered/statistics.rb b/lib/covered/statistics.rb
index dbdd90b..79ec8dd 100644
--- a/lib/covered/statistics.rb
+++ b/lib/covered/statistics.rb
@@ -28,6 +28,17 @@ def self.for(coverage)
 		class Aggregate
 			include Ratio
 			
+			# Build aggregate statistics from coverage objects.
+			# @parameter coverages [Enumerable(Covered::Coverage)] The coverage objects to summarize.
+			# @returns [Covered::Statistics::Aggregate] The aggregate statistics.
+			def self.for(coverages)
+				self.new.tap do |aggregate|
+					coverages.each do |coverage|
+						aggregate << coverage
+					end
+				end
+			end
+			
 			# Initialize empty aggregate statistics.
 			def initialize
 				@count = 0
@@ -35,8 +46,7 @@ def initialize
 				@executed_count = 0
 			end
 			
-			# Total number of files added.
-			# @returns [Integer] The number of coverage objects added.
+			# @attribute [Integer] The total number of coverage instances added.
 			attr :count
 			
 			# The number of lines which could have been executed.
@@ -67,22 +77,28 @@ def to_json(options)
 			
 			# Add coverage to these aggregate statistics.
 			# @parameter coverage [Covered::Coverage] The coverage object to add.
+			# @returns [Covered::Statistics::Aggregate] This aggregate.
 			def << coverage
 				@count += 1
 				
 				@executable_count += coverage.executable_count
 				@executed_count += coverage.executed_count
+				
+				self
 			end
 		end
 		
 		# Initialize empty coverage statistics.
 		def initialize
-			@total = Aggregate.new
+			@total = nil
 			@paths = Hash.new
 		end
 		
-		# @attribute [Covered::Statistics::Aggregate] The total aggregate statistics.
-		attr :total
+		# The total aggregate statistics.
+		# @returns [Covered::Statistics::Aggregate] The total aggregate statistics.
+		def total
+			@total ||= Aggregate.for(@paths.values)
+		end
 		
 		# @attribute [Hash(String, Covered::Coverage)] Coverage statistics indexed by path.
 		attr :paths
@@ -96,20 +112,29 @@ def count
 		# The total number of executable lines.
 		# @returns [Integer] The total executable line count.
 		def executable_count
-			@total.executable_count
+			total.executable_count
 		end
 		
 		# The total number of executed lines.
 		# @returns [Integer] The total executed line count.
 		def executed_count
-			@total.executed_count
+			total.executed_count
 		end
 		
 		# Add coverage to these statistics.
 		# @parameter coverage [Covered::Coverage] The coverage object to add.
 		def << coverage
-			@total << coverage
-			(@paths[coverage.path] ||= coverage.empty).merge!(coverage)
+			if current = @paths[coverage.path]
+				current.merge!(coverage)
+				
+				@total = nil
+			else
+				coverage = @paths[coverage.path] = coverage.dup
+				
+				@total << coverage if @total
+			end
+			
+			self
 		end
 		
 		# Get coverage for the given path.
@@ -124,7 +149,7 @@ def [](path)
 		def as_json
 			{
 				total: total.as_json,
-				paths: @paths.map{|path, coverage| [path, coverage.as_json]}.to_h,
+				paths: paths.map{|path, coverage| [path, coverage.as_json]}.to_h,
 			}
 		end
 		
@@ -151,7 +176,7 @@ def to_json(options)
 		# Print a human-readable coverage summary.
 		# @parameter output [IO] The output stream.
 		def print(output)
-			output.puts "#{count} files checked; #{@total.executed_count}/#{@total.executable_count} lines executed; #{@total.percentage.to_f.round(2)}% covered."
+			output.puts "#{count} files checked; #{total.executed_count}/#{total.executable_count} lines executed; #{total.percentage.to_f.round(2)}% covered."
 			
 			if self.complete?
 				output.puts "🧘 #{COMPLETE.sample}"
diff --git a/test/covered/coverage.rb b/test/covered/coverage.rb
new file mode 100644
index 0000000..692ca4c
--- /dev/null
+++ b/test/covered/coverage.rb
@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+# Released under the MIT License.
+# Copyright, 2018-2025, by Samuel Williams.
+
+require "covered/coverage"
+
+describe Covered::Coverage do
+	let(:source) {Covered::Source.new("foo.rb")}
+	let(:coverage) {subject.new(source, [nil, 1], 1 => ["covered"])}
+	
+	it "can be duplicated" do
+		copy = coverage.dup
+		
+		expect(copy.equal?(coverage)).to be == false
+		expect(copy.source.equal?(coverage.source)).to be == false
+		expect(copy.counts).to be == coverage.counts
+		expect(copy.counts.equal?(coverage.counts)).to be == false
+		expect(copy.annotations).to be == coverage.annotations
+		expect(copy.annotations.equal?(coverage.annotations)).to be == false
+		expect(copy.annotations[1].equal?(coverage.annotations[1])).to be == false
+	end
+	
+	it "does not share mutable state with duplicates" do
+		copy = coverage.dup
+		
+		copy.mark(2, 1)
+		copy.annotate(1, "copy")
+		copy.path = "copy.rb"
+		
+		expect(coverage.path).to be == "foo.rb"
+		expect(coverage.counts).to be == [nil, 1]
+		expect(coverage.annotations).to be == {1 => ["covered"]}
+	end
+end
diff --git a/test/covered/statistics.rb b/test/covered/statistics.rb
index 2cc5d11..c60f6fe 100644
--- a/test/covered/statistics.rb
+++ b/test/covered/statistics.rb
@@ -58,4 +58,108 @@ def before
 			expect(statistics).not.to be(:complete?)
 		end
 	end
+	
+	with "after adding overlapping coverage" do
+		let(:complete_coverage) {Covered::Coverage.new(source, [nil, 1, 1])}
+		let(:partial_coverage) {Covered::Coverage.new(source, [nil, 1, 0])}
+		
+		def before
+			statistics << complete_coverage
+			statistics << partial_coverage
+			super
+		end
+		
+		it "merges coverage for the same path" do
+			expect(statistics.count).to be == 1
+			expect(statistics.executable_count).to be == 2
+			expect(statistics.executed_count).to be == 2
+		end
+		
+		it "is complete" do
+			expect(statistics).to be(:complete?)
+		end
+	end
+	
+	with "after reading total before adding coverage" do
+		let(:partial_coverage) {Covered::Coverage.new(source, [nil, 1, 0])}
+		let(:complete_coverage) {Covered::Coverage.new(source, [nil, 0, 1])}
+		let(:other_coverage) {Covered::Coverage.new(Covered::Source.new("bar.rb"), [nil, 1])}
+		
+		it "adds new paths to cached totals" do
+			statistics << partial_coverage
+			
+			total = statistics.total
+			
+			statistics << other_coverage
+			
+			expect(statistics.total).to be_equal(total)
+			expect(statistics.count).to be == 2
+			expect(statistics.executable_count).to be == 3
+			expect(statistics.executed_count).to be == 2
+		end
+		
+		it "invalidates cached totals" do
+			statistics << partial_coverage
+			
+			total = statistics.total
+			
+			expect(statistics.count).to be == 1
+			expect(statistics.executable_count).to be == 2
+			expect(statistics.executed_count).to be == 1
+			expect(statistics.total).to be_equal(total)
+			
+			statistics << complete_coverage
+			
+			expect(statistics.total).not.to be_equal(total)
+			expect(statistics.count).to be == 1
+			expect(statistics.executable_count).to be == 2
+			expect(statistics.executed_count).to be == 2
+		end
+	end
+	
+	with "after adding coverage" do
+		let(:coverage) {Covered::Coverage.new(source, [nil, 1])}
+		
+		it "does not share mutable state with the original coverage" do
+			statistics << coverage
+			
+			coverage.mark(2, 1)
+			coverage.path = "bar.rb"
+			
+			expect(statistics.count).to be == 1
+			expect(statistics["foo.rb"].counts).to be == [nil, 1]
+			expect(statistics.executable_count).to be == 1
+			expect(statistics.executed_count).to be == 1
+		end
+	end
+end
+
+describe Covered::Statistics::Aggregate do
+	let(:source) {Covered::Source.new("foo.rb")}
+	let(:other_source) {Covered::Source.new("bar.rb")}
+	
+	with "multiple coverage objects" do
+		let(:complete_coverage) {Covered::Coverage.new(source, [nil, 1, 1])}
+		let(:other_coverage) {Covered::Coverage.new(other_source, [nil, 0])}
+		let(:aggregate) {subject.for([complete_coverage, other_coverage])}
+		
+		it "summarizes coverage" do
+			expect(aggregate.count).to be == 2
+			expect(aggregate.executable_count).to be == 3
+			expect(aggregate.executed_count).to be == 2
+		end
+	end
+	
+	with "an aggregate" do
+		let(:coverage) {Covered::Coverage.new(source, [nil, 1])}
+		let(:aggregate) {subject.for([coverage])}
+		
+		it "can add coverage" do
+			aggregate << Covered::Coverage.new(other_source, [nil, 0])
+			
+			expect(aggregate.count).to be == 2
+			expect(aggregate.executable_count).to be == 2
+			expect(aggregate.executed_count).to be == 1
+		end
+	end
 end