diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8b4559a..1860802 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,7 @@
 ## Version `v0.6.8`
 
 * ![Feature][badge-feature] Support `clamp` and `clamp!` ([#208])
+* ![Maintenance][badge-maintenance] Remove internal set type `DuplicateVector` ([#209])
 
 ## Version `v0.6.7`
 
@@ -84,6 +85,7 @@
 [badge-maintenance]: https://img.shields.io/badge/maintenance-gray.svg
 [badge-docs]: https://img.shields.io/badge/docs-orange.svg
 
+[#209]: https://github.com/adrhill/SparseConnectivityTracer.jl/pull/209
 [#208]: https://github.com/adrhill/SparseConnectivityTracer.jl/pull/208
 [#205]: https://github.com/adrhill/SparseConnectivityTracer.jl/pull/205
 [#204]: https://github.com/adrhill/SparseConnectivityTracer.jl/pull/204
diff --git a/src/SparseConnectivityTracer.jl b/src/SparseConnectivityTracer.jl
index 8ecd652..bfa4701 100644
--- a/src/SparseConnectivityTracer.jl
+++ b/src/SparseConnectivityTracer.jl
@@ -11,7 +11,6 @@ using FillArrays: Fill
 
 using DocStringExtensions: DocStringExtensions, TYPEDEF, TYPEDFIELDS
 
-include("settypes/duplicatevector.jl")
 include("settypes/recursiveset.jl")
 include("settypes/sortedvector.jl")
 
diff --git a/src/patterns.jl b/src/patterns.jl
index 5bb02b1..7a1ec1b 100644
--- a/src/patterns.jl
+++ b/src/patterns.jl
@@ -117,7 +117,7 @@ function union_product!(
 end
 
 # Some custom set types don't support `push!`
-for S in (:DuplicateVector, :SortedVector, :RecursiveSet)
+for S in (:SortedVector, :RecursiveSet)
     @eval function union_product!(
         hessian::$S{Tuple{I,I}}, gradient_x::$S{I}, gradient_y::$S{I}
     ) where {I<:Integer}
diff --git a/src/settypes/duplicatevector.jl b/src/settypes/duplicatevector.jl
deleted file mode 100644
index c334ed1..0000000
--- a/src/settypes/duplicatevector.jl
+++ /dev/null
@@ -1,42 +0,0 @@
-"""
-    DuplicateVector
-
-Vector that can have duplicate values, for which union is just concatenation.
-"""
-struct DuplicateVector{T} <: AbstractSet{T}
-    data::Vector{T}
-
-    DuplicateVector{T}(data::AbstractVector) where {T} = new{T}(convert(Vector{T}, data))
-    DuplicateVector{T}(x) where {T} = new{T}([convert(T, x)])
-    DuplicateVector{T}() where {T} = new{T}(T[])
-end
-
-Base.show(io::IO, dv::DuplicateVector) = print(io, "DuplicateVector($(dv.data))")
-
-function Base.show(io::IO, ::MIME"text/plain", dv::DuplicateVector)
-    return print(io, "DuplicateVector($(dv.data))")
-end
-
-Base.eltype(::Type{DuplicateVector{T}}) where {T} = T
-Base.length(dv::DuplicateVector) = length(collect(dv))  # TODO: slow
-Base.copy(dv::DuplicateVector{T}) where {T} = DuplicateVector{T}(dv.data)
-
-function Base.union!(a::S, b::S) where {S<:DuplicateVector}
-    append!(a.data, b.data)
-    return a
-end
-
-function Base.union(a::S, b::S) where {S<:DuplicateVector}
-    return S(vcat(a.data, b.data))
-end
-
-Base.collect(dv::DuplicateVector) = unique!(dv.data)
-
-Base.iterate(dv::DuplicateVector)             = iterate(collect(dv))
-Base.iterate(dv::DuplicateVector, i::Integer) = iterate(collect(dv), i)
-
-function product(a::DuplicateVector{T}, b::DuplicateVector{T}) where {T}
-    return DuplicateVector{Tuple{T,T}}(
-        vec(collect((i, j) for i in a.data, j in b.data if i <= j))
-    )
-end
diff --git a/test/brusselator.jl b/test/brusselator.jl
index 7b1597e..8f726d2 100644
--- a/test/brusselator.jl
+++ b/test/brusselator.jl
@@ -2,7 +2,6 @@ using ADTypes
 using ADTypes: AbstractSparsityDetector
 using ReferenceTests
 using SparseConnectivityTracer
-using SparseConnectivityTracer: DuplicateVector, RecursiveSet, SortedVector
 using SparseConnectivityTracerBenchmarks.ODE: Brusselator!
 using Test
 
diff --git a/test/flux.jl b/test/flux.jl
index afdb1d9..5ea643b 100644
--- a/test/flux.jl
+++ b/test/flux.jl
@@ -3,7 +3,6 @@ using ADTypes: AbstractSparsityDetector
 using Flux: Conv, relu
 using ReferenceTests
 using SparseConnectivityTracer
-using SparseConnectivityTracer: DuplicateVector, RecursiveSet, SortedVector
 using Test
 
 # Load definitions of GRADIENT_TRACERS, GRADIENT_PATTERNS, HESSIAN_TRACERS and HESSIAN_PATTERNS
diff --git a/test/settypes/correctness.jl b/test/settypes/correctness.jl
index 11a8288..35f61bb 100644
--- a/test/settypes/correctness.jl
+++ b/test/settypes/correctness.jl
@@ -1,10 +1,8 @@
 using SparseConnectivityTracer
-using SparseConnectivityTracer: DuplicateVector, RecursiveSet, SortedVector, product
+using SparseConnectivityTracer: RecursiveSet, SortedVector, product
 using Test
 
-@testset "$S" for S in (
-    BitSet, Set{Int}, DuplicateVector{Int}, RecursiveSet{Int}, SortedVector{Int}
-)
+@testset "$S" for S in (BitSet, Set{Int}, RecursiveSet{Int}, SortedVector{Int})
     x = S.(1:10)
     y = (x[1] ∪ x[3]) ∪ (x[3] ∪ ((x[5] ∪ x[7]) ∪ x[1]))
 
diff --git a/test/tracers_definitions.jl b/test/tracers_definitions.jl
index f7d6803..f770422 100644
--- a/test/tracers_definitions.jl
+++ b/test/tracers_definitions.jl
@@ -1,13 +1,12 @@
 using SparseConnectivityTracer: AbstractTracer, GradientTracer, HessianTracer, Dual
 using SparseConnectivityTracer: IndexSetGradientPattern
 using SparseConnectivityTracer: IndexSetHessianPattern, DictHessianPattern
-using SparseConnectivityTracer: DuplicateVector, RecursiveSet, SortedVector
+using SparseConnectivityTracer: RecursiveSet, SortedVector
 using SparseConnectivityTracer: Shared, NotShared
 
 GRADIENT_PATTERNS = (
     IndexSetGradientPattern{Int,BitSet},
     IndexSetGradientPattern{Int,Set{Int}},
-    IndexSetGradientPattern{Int,DuplicateVector{Int}},
     IndexSetGradientPattern{Int,SortedVector{Int}},
     IndexSetGradientPattern{Int,RecursiveSet{Int}},
 )
@@ -19,9 +18,6 @@ HESSIAN_PATTERNS_SHARED = (
 HESSIAN_PATTERNS_NOTSHARED = (
     IndexSetHessianPattern{Int,BitSet,Set{Tuple{Int,Int}},NotShared},
     IndexSetHessianPattern{Int,BitSet,Set{Tuple{Int,Int}},NotShared},
-    IndexSetHessianPattern{
-        Int,DuplicateVector{Int},DuplicateVector{Tuple{Int,Int}},NotShared
-    },
     IndexSetHessianPattern{Int,SortedVector{Int},SortedVector{Tuple{Int,Int}},NotShared},
     # TODO: test on RecursiveSet
     DictHessianPattern{Int,BitSet,Dict{Int,BitSet},NotShared},