mod_adhoc_dataforms_demo: Fix duplicate field prevention It's supposed to only include each type of form field once per form at most but it didn't note which fields it had added already. No idea what the probability was anyways, probably pretty low.

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summary: Allow server administrators to probe any user
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This module lets server administrators send `<presence type="probe"/>`
to any local user and receive their presence in response, bypassing
roster checks.