generate unique primary key for all clients from the local database and NOT the server
If there are no natural way of identifying a registration, you may want to have a look at GUID:

https://en.wikipedia.org/wiki/Globally_unique_identifier

The upside is that there are plenty of libraries dealing with them. The downside is that they occupy space which may lead to higher B-Tree indexes, i.e. additional I/O. More read I/O is also required because GUID's are evenly distributed, when you read a page the likelihood that you get a related row is low which causes additional pages to be read.

Another option is to add another attribute representing the host and use this in combination with your sequence as an identifier. 

primary key(host_id, seq_id)


Make sure that no two host uses the same host_id.

Yet a third option is to partition the seq_id for different hosts. I.e. host 1 uses seq_id 1..n, host 2 uses seq_id n+1..2n and so forth. Not an alternative for the heart of fainted ;-)