When giving the print commando for a merge letter, a window pops up with the
question: 
  " Your doc contains address database fields. Do you want to 
    print a form letter? "
The window offers a checbox: 
  " Do not show this warning again."

However, having checked that box, there is no posibility to get the mail merge
window, which is very usefull for some functions.

Futhermore, the option has effect on all documents with  database fields.
And it is not possible in the GUI, to undo the option

The only way to get the pop-up window back, is editing the prop
oor:name="AskForMerge" in the file Writer.xcu in the user registry path. 

This defect already exists for a long time.
(I just now made time to check it in a recent version, not short before a new
release.)