since followed by Past perfect or Past simple

[allthewayanime]

So I have this sentence and I want to know why the past perfect is used after since.Is there a difference if we use the past simple or the past perfect after since?How can I know which one is the best to use?

'Two weeks had passed since they had returned from their holiday and Leslie decided to tell Jim how she felt.'

[emsr2d2]

So I have this sentence and I want to know why the past perfect is used after since.Is there a difference if we use the past simple or the past perfect after since?How can I know which one is the best to use?

'Two weeks had passed since they had returned from their holiday and Leslie decided to tell Jim how she felt.'

Two weeks had passed since they returned from their holiday ...
Two weeks had passed since they had returned from their holiday ...

The first sounds more natural. You could always avoid the issue by saying:

Two weeks had passed since their return from their holiday ...

[allthewayanime]

But how can I know when since should be followed by the past simple or by the past perfect?