Avoiding duplicate entries in a Sharpoint list

Question

We need the following function. We have a phone column, in which the same phone number can appear several times. However, one of the multiple phone numbers must be the leading one. We want to achieve this with an additional column in which the same telephone numbers are given the value 0 in the column, but the leading telephone number should be given the 1. The 1 must not appear twice in the same telephone number. How can I implement this in a Sharepoint list so that when entering the same phone number, the 1 is not assigned twice?

phone Column, selector
123456 - 0
123456 - 0
123456 - 1 (this is the leading number)
123456 - 0
No other entry could have the 1 for the number 123456.

Answer 1

Hi @SharepointNick ,
According to your description, your requirement is more related to Power Apps. Therefore, we suggest that you can go to the Microsoft Power Apps Community for professional help.
Thank you for your support and understanding.

Thanks,
Yanli Jiang

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Answer 2

do you use online or on-prem version? In on-prem such validation may be done by implementing custom even receiver and attaching it to target list: it may check that leading number is only one and set Cancel = true otherwise. See example e.g. here: Developing an Event Receiver for the ItemUpdating event.