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Waiver is applied to your J1 status, in other words, your previous and current IAP-66, DS2019 forms, not physical visa stamp. Therefore, you can get mutiple visa stamps as long as you are in the period listed in the same DS2019 form. If you apply another DS2019 to prolong your J1 period after you get approval from DOS, you have to get a second waiver, no matter you have applied a visa stamps or not.