How to generate 8 bytes unique id from GUID?

Question

I try to use long as unique id within our C# application (not global, and only for one session) for our events. Do you know if the following will generate an unique long id?

Answer 1

Yes, this will be most likely unique but since the number of bits are less than GUID, the chance of duplicate is more than a GUID - although still negligible.

Anyway, GUID itself does not guarantee uniqueness.