Lock a table

Locking a table prevents two users to update a table at same time. 

Using locks we can prevent data replications. 

LOCK TABLE employees
   IN EXCLUSIVE MODE;

This exclusive mode enables the user to look into the rows but can update it.

And this command for remote link tables.

LOCK TABLE employees@DB

IN SHARE MODE 

The alternative for this is to use sequence in your table.It will also prevent data duplications.