Dear Yahya,
Wa alaikum assalam
A 'khalifah' does not only mean a viceroy such as in verse 38:26 or as discussed in article [1] below. A 'khalifah' can also simply mean a successor. A people / generation that succeed another. One that come after another.
Please also kindly see how the verb 'khalafa' from the same root means to succeed in verses 7:169, 7:142 or 19:59.
It is also not surprising that more than one person or people can be appointed viceroys in different parts of the world. Surely one would accept that there are different leaders / kings exercising authority and power in different parts of the world.
The same word ‘khalifah’ is also used for humans who are 'viceroys' on the earth (6:165; 35:39, 2:30). There is nothing fantastic / remarkable about this too.
Also it appears that are you misquoting verse 7:59. I am assuming you were referring to verse 7:74 instead.
If you kindly read the verses again with the above in mind, the difficulty in your mind will be removed, God willing.
Regards,
Joseph