I have an input file which looks like this:
601 a
602 a
603 a
601 b
610 c
615 c
603 d
601 d
612 d

I need the utput to look like this

601 a 602 603
602 a 601 603
603 a 601 602
601 b
610 c 615
615 c 610
603 d 601 612
601 d 603 612
612 d 601 603

Any idea how this can be done?

Regards
Wah

Firstly, please create file below:

a 601 602 603
b 601
c 610 615
d 601 603 612

Secondary, please use "join" command:

a 601 601 602 603
a 602 601 602 603
a 603 601 602 603
b 601 601
c 610 610 615
c 615 610 615
d 601 601 603 612
d 603 601 603 612
d 612 601 603 612

Thirdly, please replace 1st word and 2nd word.

Forthly, please delete same word as 1st word.

It's my idea. One step is easy.