>> I tried to use project-find-regexp more often than rgrep,
>> but unfortunately xref still has a fundamental flaw:
>> 0. emacs -Q
>> 1. M-x project-find-regexp RET regexp RET
>> 2. The output buffer *xref* contains duplicate lines
>>     when regexp is found on the same line several times,
>>     each duplicate output line has separate highlighting
>>     for every regexp occurrence.
>
> I don't know how "fundamental" it is, but indeed, it's somewhat of
> a drawback. Suggestions for improving it (API change and/or implementation
> change) are welcome.

Here is the patch that makes the broken project-find-regexp usable: