[MU4] Fix #311986: handle duplicate voltas

[jeetee]

Resolves: https://musescore.org/en/node/311986

Cherry pick of PR #6810 from 3.x to master

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[vpereverzev]

https://github.com/musescore/MuseScore/pull/6904/checks?check_run_id=1436848735
code style check failure, should be easy to fix

[jeetee]

I don't have uncrustify locally, and QtCreator seemed happy. Especially those multiline function arguments is what it does by default when using the Qt codestyle.
Do we have a style file for QtCreator as we did for 3.x ?

I'll apply those changes but FWIW I'm rather against the requirement to place the pointer indicator concatenated with the type name (being fully aware that this is somewhat a "holy war" point for many). My arguments against it are:

1. A T* itself isn't a type, the '*' itself is the type
2. It breaks style as soon as const comes into play, something that is very important to convey code intent
3. It completely break style when volatile comes into play (admittedly, as we're not doing hardware interfacing as much, this is probably a moot worry in the MS codebase).
4. (I might be biased here because of this) Coding safety standards such as the Barr standard call for the space to be there as a lexical safety rule.
   Note that this only applies to the pointer type, not to the reference indicator, which should be concatenated.

[jeetee]

@vpereverzev ready