GBIF gets data from (as I write this) over 43,000 different sources. Some are static one time loads, like a museum collection inventory (or at least infrequently updated) others are updated frequently.
https://www.gbif.org/dataset/search
Its not simply a matter of dupication from individuals who may use multiple sites, its also from multiple individuals submitting the same thing. You are a birder in Canada, but I dont know where so hopefully the example makes sense. How many different people reported that famous Great Kiskadee in Rondeau this fall ? Let alone how many different folks report something that is not a one in a lifetime experience (likely), but rather simply rare, for instance the Black-throated Grey Warbler that visited a park near my home in Ontario this fall.
And then GBIF has to deal with that globally across 40000+ providers, with no time limit on when something may be submitted. Not easy.