MongoDB - multi wirte data to same document
With (multiple) updates to (single) document, first come, first served. So, that client who is first (in time) get lock to that document, others who come later, will get "ticket" (queue number) and will wait their turn to update that document. 

So, update 2 will override field1 value and add field2, update 3 then will override field2 value..