Every collector has his own defined rules. The rules for collecting beer glasses are not written in some "Book of rules" published somewhere. When you are starting a hobby you are defining the rules, which can be changed (adapted) afterwards if necessary. Here are the rules for my beer glasses collection:
The collection must have only beer glasses (not ceramics, not metal, or any other material)
The capacity of the beer glass must be equal or greater to 0.4 liters
The beer glasses must be branded (with logo of the beer, or brewery). The logo can be printed on the glass; it can be etched on the glass; it can be single sided (one logo), or double sided (two logos); etc.
If there are 2 beer glasses into the beer glasses collection from one beer brand that has exactly the same shape, but different logo, they are not considered as doubles.