Replace all multi-blankspaces (more than one spaces/tabs) in file with mono-blankspace (one space):

sed 's/[<space>][<tab>][<space>][<tab>]*/ /g' filename

can also be:

 sed 's/[<space>][<tab>][<space>][<tab>]*/ /g' filename > newfilename

note: g in the end means GLOBAL

In the above samples, <space> means ‘ ‘ (empty space) and <tab> means ‘      ‘ (tab key press).