Lots of possibilities:

One possibility:

Start with 12
Tack 1 2 onto 12. Get 122.
Tack 2 4's onto 12. Get 1244
Tack 3 6's onto 12. Get 1244666.


Another possibility:

Replace the last digit, 2, by 1 times 2, which is 2, and tack that same number
onto the end.  Get 122.
Replace the last digit, 2, by 2 times 2, which is 4, and tack that same number
onto the end.  Get 1244.
Replace the last digit, 4, by 3 times 4, which is 12, and tack that same number
onto the end. Get 1241212.

Another possibility

Or you could do it another way.






Or it could be twice this sequence:

6,61,622,6333,64444,655555

For when you double them you get

12,122,1244,12666,128888,1311110 

Many, many possibilities.  Which is correct?  They all are.

Edwin