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i) you cannot add TC users to any external sdr organization ( only exception is Flight services )
ii) you cannot add any a user to any SDR organization who is already assigned to another SDR organization ( you will occasionally more than 1 SDR organization. You will frequently need to create a 2nd userid for some people ), otherwise we would remove that user from the 1st account, then and add them to the 2nd ( this will not be possible if they have submitted SDRs)
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