Double prepositions are okay, but i believe in your sentences that: 1. “back” is a particle in the phrase “pay back” 2. “in” is a particle for the phrase “called in” so neither are prepositions. A few examples are: “Water shot -out of- the nozzle.” “The sound came -from in- that room.”

Hi Davood, Yes, it's fine to do that. Your first sentence is perfect. The second sounds a little strange because you used 'at' and 'to call in' implies movement from one place to another. Do you mean 'I called him in to his office', or 'I called him in from his office'? Either of these is fine.