When any step in a tour uses the attachTo property for positioning the pop up, the back button shows both the next and the previous slide at the same time.
For steps which do not use the attachTo property, you can navigate forward and back normally without any problem.

Steps to Reproduce:

1. Load the items Wikidata tour - https://www.wikidata.org/w/index.php?title=Q16943273&tour=wbitems
2. Verify that the back button works perfectly up to and including the "Item Pages" step (step number 5)
3. Once you reach the "Labels" step (step number 6), press the back button

Actual Results:

After clicking the back button on step 6 (the first step to include an attachTo property), the pop ups for both step 5 and step 7 are shown at the same time.

The same can be seen in subsequent steps in the tour, as well as on step 3 onward in the statements tour

Expected Results:

Only the previous step should be displayed when clicking the back button, step 5 in this case.
Step 7 should only be revealed after clicking the forward button (or completing an action that skips to the next slide)