How To Prevent Duplicate URL Entries in the Cell in a Column?

Dear Experts,

I tried to solve above problem using Data Validation formula but it does not help as per my wish. Because data validation does not work if somebody copy paste the duplicate values,it works when somebody types duplicate values in next column. So if I want to enter lot of urls to a column then it is completely impossible to type so must have to copy paste from the browser. I have attached a sample sheet and kindly requested you all to solve this problem either using formula or using Data validation in sample 1 and sample 2 respectively.

Thanking you for your help in advance.

Regards