deduplicate_knots!() doesn't deduplicate?

[dodoplus]

check:

(@v1.6) pkg> st Interpolations
      Status `~/.julia/environments/v1.6/Project.toml`
  [a98d9a8b] Interpolations v0.13.4

julia> xx = [3.6321974345434977, 3.769939196468377, 3.7933987387602532, 3.7933987387602532, 3.7933987387602537, 3.864656907914981];

julia> Interpolations.deduplicate_knots!(xx);

julia> allunique(xx)
false

[mkitti]

Try applying it twice.

julia> xx = [3.6321974345434977, 3.769939196468377, 3.7933987387602532, 3.7933987387602532, 3.7933987387602537, 3.864656907914981]
6-element Vector{Float64}:
 3.6321974345434977
 3.769939196468377
 3.7933987387602532
 3.7933987387602532
 3.7933987387602537
 3.864656907914981

julia> Interpolations.deduplicate_knots!(xx)
6-element Vector{Float64}:
 3.6321974345434977
 3.769939196468377
 3.7933987387602532
 3.7933987387602537
 3.7933987387602537
 3.864656907914981

julia> allunique(xx)
false

julia> Interpolations.deduplicate_knots!(xx)
6-element Vector{Float64}:
 3.6321974345434977
 3.769939196468377
 3.7933987387602532
 3.7933987387602537
 3.793398738760254
 3.864656907914981

julia> allunique(xx)
true

The issue is that

julia> xx = [3.6321974345434977, 3.769939196468377, 3.7933987387602532, 3.7933987387602532, 3.7933987387602537, 3.864656907914981]
6-element Vector{Float64}:
 3.6321974345434977
 3.769939196468377
 3.7933987387602532
 3.7933987387602532
 3.7933987387602537
 3.864656907914981

julia> nextfloat(xx[4]) == xx[5]
true

[dodoplus]

this is still a bug in deduplicate_knots!()
It assumes there are no consecutive floats in the knots, but if duplicate knots are possible, there's no reason to assume almost duplicates are just (or more) likely. For example, say I have an exponential function and want to interpolate its inverse.

[mkitti]

Feel free to propose a fix. As this point, throwing an error is reasonable since it is far from clear what the user expects in this situation. Should we drop a knot?

[dodoplus]

What about changing:

        if knots[i] == last_knot
            @inbounds knots[i] = nextfloat(knots[i-1])
        else
            last_knot = @inbounds knots[i]
        end

to

        if knots[i] == last_knot
            @inbounds knots[i] = nextfloat(knots[i-1])
        end
        last_knot = @inbounds knots[i]

[mateuszbaran]

I've run into the same problem and made a PR: #468 .