I filed a provisional patent app about a year ago. Seven months later I refiled the exact same provisional. Does the second filing delete or get rid of my original filing date? Do I now only have the second filing to rely on as a priority date? Or is the original first provisional priority date still valid?
The second provisional filing does not affect the validity of the first one at all. You could freely claim priority to just the first one if you wanted.
In addition, in the US, you could validly file a non-provisional application based on the second provisional only, at any point within 12 months of the second provisional's filing date if you wanted. The key risk though is that there is some disclosure between the two provisionals.
However, for non-US Conventional applications, assuming you filed the second provisional before the first was abandoned, the 12 month Convention deadline still runs from the first provisional, even if you only claim priority to the second. That is, it would be an invalid priority claim if you only claimed priority to the second provisional 12 months after its filing date (since that's outside the 12 months of the first provisional).
Could you abandon the first provisional subsequently to use the second provisional for international priority?
No. The Paris Convention art 4(C)(4) expressly provides that a second filed application can only be used for priority if:
at the time of filing the subsequent application, the said previous application has been withdrawn, abandoned, or refused, without having been laid open to public inspection and without leaving any rights outstanding, and if it has not yet served as a basis for claiming a right of priority.
Thus, once the second one has been filed, there is nothing you can do to recover it.