Use this structure to say that the other person's action is the timing for your result: When you arrive, call me. In this standard time-clause structure, only one 'when' is needed to join the two clauses. A repeated form like When you arrive, when you can, call me does appear in casual speech, where repeating 'when' builds playful suspense, but that's a separate stylistic pattern, not part of this structure.
If their action is the starting point and yours depends on it: When you finish the report, send it to me. Both clause orders work.
You're telling someone what will happen once they do something
Giving instructions to a colleague: When you arrive, call me. Their action comes first, and your action depends on it. Put a comma after the opening clause when it starts the sentence.
Reassuring a friend before a trip: When you land, I'll pick you up from the airport. One future event triggers another, so a single when is enough in this structure. You may hear people repeat when for emphasis in casual speech, but in a neutral sentence like this it isn't needed.
Contrasting case: if the two actions happen at the same time rather than one triggering the other, this pattern isn't what you need. When you cook, I read describes overlapping habits, so use While you cook, I read or restate the overlap in plain wording.
One detail changes everything: the comma. When the when clause opens the sentence, it's followed by a comma: When you land, call me. Flip the order and the comma disappears: Call me when you land. Same meaning, same two clauses, only the punctuation shifts.
So you've got a real choice here. Put the condition first if you want it to sound like the starting point, with a comma after it. Put it last if the main instruction matters more. Both are correct; just don't keep the comma when when comes second.
When the when clause comes second, no comma is needed. The rule doesn't change which words you use, only the comma. So When you land, call me keeps its comma, but Call me when you land drops it, because a when clause at the end of a sentence isn't separated by a comma. And the structure needs only one when: you state the condition once, either before or after the main clause. If your sentence reads Call me, when you land, the comma is the one thing to remove.