This sounds like a classic case for an optimistic strategy e.g.
Optimistic concurrency control (OCC) is a concurrency control method
for relational database management systems that assumes that multiple
transactions can complete without affecting each other, and that
therefore transactions can proceed without locking the data resources
that they affect. Before committing, each transaction verifies that no
other transaction has modified its data. If the check reveals
conflicting modifications, the committing transaction rolls back
Perhaps maintaining record version numbers.
e.g. each record would maintain a record version number. When a client
receives the client, it would receive the version number too. Upon
modification the server compares the record number with that in the
database, and if not changed, then a new record is written with an
incremented version number.
A second client would then come in with the same record number (i.e. the
record is being updated by both clients). This time the server would
recognise that the version number has changed, and reject the update.
Note that various implementation options exist. The database can
implement this using record numbers or timestamps. The database can
maintain a history of edits, or simply the latest version of the record.