I think following aricle 'Primary Keys' should be of help http://www.craigberntson.com/Articles/kb006.htm

>I'm in the process of developing a multiuser application and am looking for some advice on primary key generation.. In single apps I used max + 1 for the primary key which works fine in a single user environment, but could cause problems in a multiuser environment.. I had the idea of a table which holds the next primary key for all of the tables.. When you want a new key you exclusivly lock the table, get the key, update the table and the release the key table. Any advice on how other people generate unique primary keys would be appriciated...
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> Thanks