Combine lines with matching first field

Question

For a few years, I often have a need to combine lines of (sorted) text with a matching first field, and I never found an elegant (i.e. one-liner unix command line) way to do

Answer 1

I think this one do the job

 awk -F':' '$1!=a{if(b);print b;b=""}a=$1{$1="";if(!b)b=a;b=b$0}END{print b}' infile