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The solution is to use an associative array to store what you've already seen:

```bash
#!/bin/bash

unset seen
declare -A seen

for arg in "$@"
do
    if [[ -z "${seen[$arg]}" ]]
    then
        echo "Doing something omplicated with $arg"
        seen["$arg"]=1
    fi
done
```
The `unset seen` is just in case the caller had an exported variable named `seen`. The `declare -A seen` tells bash to treat `seen` as an associative array, that is, it takes arbitrary strings as index.

The loop then tests for each argument whether it has not yet been seen (in which case `"${seen[arg]}"` is empty). If so, it processes it and then marks it as seen by storing something in `seen["$arg"]` (what it stores doesn't really matter as long as it is not empty; if you want, you can store additional information about the argument here, e.g. the result of processing this argument).