TouchDown for Android will resync the mailbox only if you unselect and reselect the folder OR the server tells it to resync completely.
The only known situation where that happens (excluding situations where the email ActiveSync client is having issues, which we cannot fix) is if you are using a device which is changing the device IDs frequently, or if you are using two devices with the same device ID against the same account (or syncing the same accounts to two different profiles on the same device).
Imported Document ID: HOWTO100434
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