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The sed FAQ
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5.2. I'm using 'p' to print, but I have duplicate lines sometimes.

Sed prints the entire file by default, so the 'p' command might cause the duplicate lines. If you want the whole file printed, try removing the 'p' from commands like 's/foo/bar/p'. If you want part of the file printed, run your sed script with -n flag to suppress normal output, and rewrite the script to get all output from the 'p' comand.

If you're still getting duplicate lines, you are probably finding several matches for the same line. Suppose you want to print lines with the words "Peter" or "James" or "John", but not the same line twice. The following command will fail:

     sed -n '/Peter/p; /James/p; /John/p' files

Since all 3 commands of the script are executed for each line, you'll get extra lines. A better way is to use the 'd' (delete) or 'b' (branch) commands, like so (with GNU sed):

     sed '/Peter/b; /James/b; /John/b; d' files          # one way
     sed -n '/Peter/{p;d;};/James/{p;d;};/John/p' files  # a 2nd way
     sed -n '/Peter/{p;b;};/James/{p;b;};/John/p' files  # a 3rd way
     sed '/Peter\|James\|John/!d' files                  # shortest way

On standard seds, these must be broken down with -e commands:

     sed -e '/Peter/b' -e '/James/b' -e '/John/b' -e d files
     sed -n -e '/Peter/{p;d;}' -e '/James/{p;d;}' -e '/John/p' files

The 3rd line would require too many -e commands to fit on one line, since standard versions of sed require an -e command after each 'b' and also after each closing brace '}'.

The sed FAQ
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   Reprinted courtesy of Eric Pement. Also available at Design by Interspire