I wasn't able to recreate this exact problem, but I think I can see what is causing it.
Until you press the <Enter> key, Excel remains in 'editing mode'. If it's still in 'editing mode' when you switch worksheets, it automatically changes the cell reference that's being edited to the sheet that you click.
The solution is to press the <Enter> key after step iv. You'll then be returned to sheet2 with the correct formula entered and no duplication.
If you need to do something else with the value (such as add, subtract, multiply, etc.), you just need to type the correct operator before switching tabs. As long as you have typed an operator, Excel will know that you are going to enter a new cell reference and won't modify the one you've already entered.
You can see more about cross-worksheet formulas in the video tutorial: Essential Skills Lesson 6-6 Create cross worksheet formulas.
I hope this is the solution to your problem, but please feel free to reply if you need any more help.
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