Concatenate Lines 'a' and 'b' for each key.

My spreadsheet has hundreds of lines. Here is a sample:

deletar.png

I would like to merge all the elements of lines 'a' and 'b' for each key index "3A~" into a single line.
The order of the elements in this new line would be: first all the elements of 'a' line (preserving original order o 'a'). THEN, all the elements of 'b' line (preserving original order of 'b').

It's the same as concatenating the 'b' line into the end of 'a' line, deleting the former 'b' line and renaming "3A~a" to "3A~".

expected result:
deletar2.png

Any help will be much appreciated!

Please send your spreadsheet