All we need is an easy explanation of the problem, so here it is.
How to solve :
I know you bored from this bug, So we are here to help you! Take a deep breath and look at the explanation of your problem. We have many solutions to this problem, But we recommend you to use the first method because it is tested & true method that will 100% work for you.
Photoshop’s line shape tool actually creates a very thin rectangle, which results in a double stroke effect when used the way the OP has described. The solution is to simply use the pen tool to create a line.
Note: Use and implement method 1 because this method fully tested our system.
Thank you 🙂