Replic From Multiple Clients: Identifier Conflicts

I think I will have a problem with Id’s when I do merge replication. The problem is, I have several clients (local shops) who can do additions to a (Customer) table. Other tables will link to that table (things that a customer buys) with their foreign key, so the id of this table is very important. Now, twice a day that table with customers is merged from all clients to the central server. (the table with things that the customer buys, does not needs to be merged as it only remains local).

Possible problem:
if we assign ranges to identifiers for each client (thus local shop), which will be the best option we found until so far, how can we then detect the doubles? (with doubles, we mean that when the same customer is added in two local shops for a first time, then we will have the same customer twice in the central database)(afterwards, when we delete one customer row in the central database, we might have a id conflict when we merge back to the clients -as a customer might have already bought some goods at the two different shops meanwhile-)

What’s the common practice to resolve this?
(we need to do Merge Replication, because during the day, our local shops are disconnected from the central server)