Използвайте distinct
In [27]: agent_ids = agents_collection.find().distinct('_id')
In [28]: agent_ids
Out[28]:
[ObjectId('553662940acf450bef638e6d'),
ObjectId('553662940acf450bef638e6e'),
ObjectId('553662940acf450bef638e6f')]
In [29]: agent_id2 = [str(id) for id in agents_collection.find().distinct('_id')]
In [30]: agent_id2
Out[30]:
['553662940acf450bef638e6d',
'553662940acf450bef638e6e',
'553662940acf450bef638e6f']