<p>If there a way to protect against concurrent modifications of the same data base entry by two or more users?</p>

<p>It would be acceptable to show an error message to the user performing the second commit/save operation, but data should not be silently overwritten.</p>

<p>I think locking the entry is not an option, as a user might use the "Back" button or simply close his browser, leaving the lock for ever.</p>

To be safe the database needs to support
transactions
.
If the fields is "free-form" e.g. text etc. and you need to allow several users to be able to edit the same fields (you can't have single user ownership to the data), you could store the original data in a variable.
When the user committs, check if the input data has changed from the original data (if not, you don't need to bother the DB by rewriting old data),
if the original data compared to the current data in the db is the same you can save, if it has changed you can show the user the difference and ask the user what to do.
If the fields is numbers e.g. account balance, number of items in a store etc., you can handle it more automatically if you calculate the difference between the original value (stored when the user started filling out the form) and the new value you can start a transaction read the current value and add the difference, then end transaction. If you can't have negative values, you should abort the transaction if the result is negative, and tell the user.
I don't know django, so I can't give you teh cod3s.. ;)