According to sample values returned by NEWID, it seems that SQL Server uses Version 4, which is based on random numbers. (See, for example, https://en.wikipedia.org/wiki/Universally_unique_identifier).
If this is true, then theoretically it is possible to get collisions on same of different servers, however “probability so small that it can normally be ignored”, “the probability to find a duplicate within 103 trillion version-4 UUIDs is one in a billion” (https://en.wikipedia.org/wiki/Universally_unique_identifier#Version_4_(random)).
Regarding other collisions caused by concurrent execution, I think that they are not possible. Documentation does not mention this, maybe because the whole SQL Server is designed for concurrent access.