Symbols in the Wolfram Language
aaaaa user‐defined symbol
Aaaaa system‐defined symbol
$Aaaa global or internal system‐defined symbol
aaaa$ symbol renamed in a scoping construct
aa$nn unique local symbol generated in a module
According to the forms of symbols, x in Unique[x] shouldn't be aaaa$$.
Workaround:
In:
Unique["x$$"]
Out:
x$$6