Update requests and contact lists

Hi,
We need to send an update request to our vendor and I set up an automated workflow to send out this update request. When testing and opening the test update request we are also able to see the contact details from other vendors in a drop down. How can I change this to only having the vendor see it's own email address and be able to update that if necessary?
Answers
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@Anja Smeets can you just remove the contacts column from the update request?
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@Ella I could do that, but it is a column we would like them to update if necessary, so removing it is not really helpful.
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@Anja Smeets I see, in that case I don't see how you can limit drop-down values in a single contact column. Unless you have a separate contact column and drop-down list for each vendor.