Double clicking on a pallet item should not add two identical elements
Since single clicking to add a pallet item was implemented there has been a steady stream of users reporting various problems caused by multiple identical overlapping and hence un-seeable items (notably voltas) being found in their scores to their surprise ("I didn't add them!"). It seems fairly certain that this is a result of the user using the old double click method to add an item.
My suggestion is that when adding pallet items, both single and double click should be interpreted as single click.
Alternatively, a check for the presence in the score pre-existing identical (i.e. identical type, and location) element should be made on any click and if such an element is found, the user should be warned that they are about to duplicate it. However, this is probably a less helpful and more difficult to implement option.