I can not figure out this one, so I turn to unix.com for help, I have a file, in which there are some lines containing continuously duplicate columns, like the following
You need to have backrefs already in the first part to make sure you are actually replacing repeats, otherwise it will simply reduce every line to one token (or two, if you require a trailing space after the replacement). Also you need to make up your mind on whether your sed requires a backslash before grouping parentheses or not.
sed 's/\([^ ]* \)\1*/\1/g' file
I used * instead of +; if your sed understands the plus, then by all means use that.