path_update.sh

#!/bin/sh --
# example of .path_dirs file:
#     /home/anddam/bin
#     /home/anddam/.local/bin

{
    # This 'config' should probably be found more nicely than this.
    cat "${1:-$HOME/.path_dirs}"
    IFS=:
    # Wordsplitting is purposefully used here over a colon.  PATH is
    # guaranteed to be separated by colons so this is reliable.
    # shellcheck disable=SC2086
    printf '%s\n' $PATH
} | {
    while read -r dir; do
        # printf "dir=%s\n" $dir
        [ -d "$dir" ] && set -- "$@" "$dir"
    done

    # printf 'arg=%s\n' "$arg"
    for arg; do
        # printf 'arg=%s\n' "$arg"
        case :$tmp: in
            *:$arg:*) ;;
            *) path=${path:+$path:}$arg
        esac

        tmp=${tmp:+$tmp:}$arg
    done

    printf '%s\n' "$path"
}