database: Avoid merging the same data twice

database: Avoid merging the same data twice

When finish splitting networks into many parts, we have
a list of subnets with the excluded subnets and merge them
together first and put them on the stack again.

This is slower than pushing it all onto the stack first
and then popping the first element.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>