Purchase order Question, same products from differnet vendors

Purchase order Question, same products from differnet vendors

I have another question with raising POs.
 
We routinely purchase items from a number of suppliers/vendors to ensure we get the best price.  It seems that when we try and raise a PO in SDP, for a product that we have previoulsy ordered from a differnet supplier/vendor (reseller), then we cant order the same product item form a differnet vendor/supplier (reseller). We get this error
 
 
 

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