I think appending, "[Complete, but not unique]????" should
serve as a cue that one can simply RET.
You mean we'd display "foo[Complete but not unique]{bar | | baz}"?
FWIW, in my own code (icomplete+.el), I have a separate user option for the
string that is displayed to indicate an exact match, as opposed to doing this
(in icomplete.el):
(and most-is-exact
(substring icomplete-separator
(string-match "[^ ]" icomplete-separator)))
The default value of the option is (string ?\u2605 ?\ ), i.e., a star. See
attached. (The number 5 shown is the total number of completions. In this
particular case all are shown.)