I want to check if [[ $var == foo || $var == bar || $var == more ]] without repeating $var n times.

The portable solution uses case: 

   # Bourne
   case "$var" in
      foo|bar|more) ... ;;
   esac

In Bash and ksh, Extended globs can also do this within a [[ command: 

   # bash/ksh -- ksh does not need the shopt
   shopt -s extglob
   if [[ $var = @(foo|bar|more) ]]; then
      ...
   fi

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