There are two types of "deduplication" on Zapier. One for Triggers and one for Actions, respectively.
Zapier relies on a few methods to make sure that your Zap does not trigger multiple times for a single "new" item.
The way Zapier does this is generally by polling, looking for new items every few minutes (your plan's "syncing interval"). Zapier stores an identifier for each item seen so that the Zap never triggers off of that item again.
If you have two Zaps looking at the same Trigger (for example, two Zaps which trigger off the same Wufoo form) both Zaps will be triggered by new items. Deduplication happens on a per-Zap basis.
Note: certain apps use webhooks ("instant") which avoid the need to deduplicate all together, the rest of the information about Trigger deduplication here does not apply for instant Triggers.
Deduplication has several important ramifications you should be aware of:
Once Zapier has seen an item for a Zap, it will never trigger again even if the item is updated in the future.
Custom Filters are applied after deduplication happens. So you can't use a custom filter to trigger when an item changes. For example, a #hashtag applied to an Evernote title one day after the note was originally created would not work.
Trigger Filters are applied before deduplication happens. You can use regular filters to catch "changes". For example, using the "new note" Evernote trigger and choosing a specific tag to watch via regular filters would trigger even if you applied the tag one day after the note was originally created.
The following Apps and Triggers have a special "steady state" post-deduplication process applied. Normally a new item will Trigger a Zap as soon as Zapier first sees it. However for certain things, that is undesirable because you might still be typing into your Evernote note, for example, when Zapier first detects it.
The steady state postprocessor will wait to Trigger a Zap until the item has stopped changing across one polling interval ("syncing interval").
Depending on the app, that can result in several outcomes if your zap is trying to create data already in that app:
Duplication: Some apps allow for duplicates and therefore the zap will create those duplicates.
Errors: Some apps will return an error message, which will be passed on to the user in their task history.
Updates: Rarely, we are able to update duplicate contacts with data from the zap. WARNING: Updating can often mean over-writing in this case, so be aware if there's information in the action app you might potentially lose.
Stop/Ignore: Rarely, the zap will attempt to add a duplicate, but the action app will recognize the duplicate and take no action.