De-duplicate three or more duplicate variables

[d-torrance]

We refactor the dedupSymbols helper function, which previously only de-duplicated up to two duplicate variables.

Before

i1 : QQ[x,x]

o1 = QQ[x ..x ]
         0   1

o1 : PolynomialRing

i2 : QQ[x,x,x]
stdio:2:1:(3): error: baseName: no base name available

After

i1 : QQ[x,x,x]

o1 = QQ[x ..x ]
         0   2

o1 : PolynomialRing

Closes: #3261

[mahrud]

@DanGrayson I think you should try asking someone to review pull requests before merging them. GitHub even gives you suggestions for reviewers based on who else has touched those lines before.

[mahrud]

This could have been fixed with a much simpler patch:

Suggested change

	counter := new MutableHashTable from applyKeys(mapping,
	i -> varlist#i, dups -> makeVars(#dups, first dups));
	i -> varlist#i, dups -> makeVars(#flatten dups, last dups));