Fix duplicate entries in input selectors

[hinerm]

This PR represents an alternative to this PR attempting to fix the duplicates earlier in the framework.

However, this does create a problem if the converters themselves consistently create new items. For example, this is what happens when running a trivial script multiple times:

//@ImagePlus in1
//@ImagePlus in2

println(in1)
println(in2)

This could be a bug in particular converters (ImageTitleToImagePlusConverter is a likely suspect) but it does raise a concern of potential behavior for other converters, too.