diff --git a/src/animations.jl b/src/animations.jl
index 33060cd..e94c884 100644
--- a/src/animations.jl
+++ b/src/animations.jl
@@ -7,10 +7,6 @@ end
 
 AnimationTrack{T}(name::String, jstype::String) where {T} = AnimationTrack(name, jstype, Pair{Int, T}[])
 
-wider_js_type(::Type{<:Integer}) = Float64  # Javascript thinks everything is a `double`
-wider_js_type(::Type{Float64}) = Float64
-wider_js_type(x) = x
-
 function Base.insert!(track::AnimationTrack, frame::Integer, value)
     i = searchsortedfirst(track.events, frame; by=first)
     if i <= length(track.events) && first(track.events[i]) == frame
@@ -22,16 +18,17 @@ function Base.insert!(track::AnimationTrack, frame::Integer, value)
 end
 
 function Base.merge!(a::AnimationTrack{T}, others::AnimationTrack{T}...) where T
-    l = length(a)
-    events = copy(a.events)
+    l = length(a.events)
     for other in others
         @assert other.name == a.name
         @assert other.jstype == a.jstype
-        events_temp = similar(events, length(events) + length(other.events))
-        mergesorted!(events_temp, events, other.events; by=first)
-        events = events_temp
+        l += length(other.events)
     end
-    a.events = unique(first, a.events) # TODO: use unique! in ≥ 1.1
+    events = similar(a.events, l)
+    mergesorted!(events, a.events, Iterators.flatten((other.events for other in others)))
+    combine!(events, by=first, combine=(a, b) -> b)
+    resize!(a.events, length(events))
+    copyto!(a.events, events)
     return a
 end
 
diff --git a/src/atframe.jl b/src/atframe.jl
index 70b7472..23fc033 100644
--- a/src/atframe.jl
+++ b/src/atframe.jl
@@ -1,3 +1,7 @@
+wider_js_type(::Type{<:Integer}) = Float64  # Javascript thinks everything is a `double`
+wider_js_type(::Type{Float64}) = Float64
+wider_js_type(x) = x
+
 function _setprop!(clip::AnimationClip, frame::Integer, prop::AbstractString, jstype::AbstractString, value)
     T = wider_js_type(typeof(value))
     track = get!(clip.tracks, prop) do
diff --git a/src/util.jl b/src/util.jl
index 07399bd..d31216e 100644
--- a/src/util.jl
+++ b/src/util.jl
@@ -1,6 +1,25 @@
 # Port of possible implementation of std::merge (second version),
 # https://en.cppreference.com/w/cpp/algorithm/merge
 # TODO: contribute to base.
+
+"""
+    mergesorted!(dest::AbstractVector, a, b; by=identity, lt=isless)
+
+Merge sorted iterators `a` and `b`, storing the result in `dest`.
+`dest` should be of the appropriate length to store all elements of `a` and `b`;
+it is not resized.
+
+For equivalent elements in `a` and `b`, the elements from `a`
+(preserving their original order) precede the elements from `b`
+(preserving their original order).
+
+Elements are compared by `(x, y) -> lt(by(x), by(y))`
+
+The behavior is undefined if `dest` overlaps `a` or `b` (though the `a` and `b`
+may overlap each other) or if `a` or `b` are not sorted.
+
+Adapted from https://en.cppreference.com/w/cpp/algorithm/merge.
+"""
 @inline function mergesorted!(dest::AbstractVector, a, b; by=identity, lt=isless)
     i = 1
     it_a = iterate(a)
@@ -31,3 +50,29 @@
     end
     return dest
 end
+
+# from https://github.com/JuliaOpt/MathOptInterface.jl/blob/d3106f434293a2aae7c664a22a30a7bd4069111a/src/Utilities/functions.jl#L390.
+# TODO: contribute to Base
+function combine!(x::AbstractVector; by=identity, keep=x->true, combine)
+    @boundscheck issorted(x; by=by) || throw(ArgumentError("Input is not sorted."))
+    if length(x) > 0
+        i1 = firstindex(x)
+        for i2 in eachindex(x)[2:end]
+            if by(x[i1]) == by(x[i2])
+                x[i1] = combine(x[i1], x[i2])
+            else
+                if !keep(x[i1])
+                    x[i1] = x[i2]
+                else
+                    x[i1 + 1] = x[i2]
+                    i1 += 1
+                end
+            end
+        end
+        if !keep(x[i1])
+            i1 -= 1
+        end
+        resize!(x, i1)
+    end
+    return x
+end
diff --git a/test/visualizer.jl b/test/visualizer.jl
index 45fd3d6..420f0dd 100644
--- a/test/visualizer.jl
+++ b/test/visualizer.jl
@@ -270,6 +270,26 @@ end
     end
 end
 
+@testset "AnimationTrack" begin
+    track1 = MeshCat.AnimationTrack{Float64}("foo", "bar")
+    @test track1.name == "foo"
+    @test track1.jstype == "bar"
+    insert!(track1, 0, 32.0)
+    @test track1.events == [0 => 32.0]
+    insert!(track1, 2, 64.0)
+    @test track1.events == [0 => 32.0, 2 => 64.0]
+    insert!(track1, 2, 65.0)
+    @test track1.events == [0 => 32.0, 2 => 65.0]
+
+    track2 = MeshCat.AnimationTrack{Float64}("foo", "bar")
+    insert!(track2, 1, 17.0)
+    insert!(track2, 2, 66.0)
+    insert!(track2, 5, 1.0)
+
+    merge!(track1, track2)
+    @test track1.events == [0 => 32.0, 1 => 17.0, 2 => 66.0, 5 => 1.0]
+end
+
 @testset "setvisible!" begin
     v = vis[:box_to_hide]
     setobject!(v, HyperRectangle(Vec(0., 0, 0), Vec(0.1, 0.2, 0.3)))