merge title and artist field

How do I mergelcombine the artist field 2 the title field?

4 compilation cd I would like to read as follows:

Title: Cantaloop
Artist: US3

Title: Cantaloop - US3

Try this, it works but if you run this twice you end up with:
%title%: "Cantaloop - US3 - US3"

Action type: Format value
Field: TITLE
Formatstring: %title% - %artist%

###EDIT###

This one works as many times as you want without appending to the %title% like above:

Action type: Format value
Field: TITLE
Formatstring: $if($and(%TITLE%,%ARTIST%),$if($eql($right(%TITLE%,$len( - %ARTIST%)), - %ARTIST%),%TITLE%,%TITLE% - %ARTIST%),%TITLE%)