I was using a procedure-block that came with a variable x. Inside of it, I introduced a couple of local variables. Every time I added a new variable in the block, the blocks I used with variable x in it suddenly became blocks with the name of the last variable I added.
Please show your relevant blocks
a short video of the behaviour might be better...
Variable names have to be unique enough to let AI2 decide which is which, give or take a scope boundary.
The same applies to procedures.
AI2 'helps' you by forming renames (adding numbers) to the clones and their references.
Give your newborn variables new names immediately to avoid confusion.
By sheer coincidence, this week I heard news stories of a US political primary candidate who was nominated to spite an opponent. The new candidate had the same name as the opponent. News at 11.
I like ScreenToGif for a free lightweight screen recorder.
...as when Patroclus wore Achilles' armor 
(more or less!)