Same surname marriage

Same surname is o.k. No problem but People within the gotra are regarded as siblings and marrying such a person would be thought of as taboo. In almost all Hindu families, marriage within the same gotrais not encouraged or practised since they are believed to be descended from thesame family. Marriages between opposite gotras are therefore encouraged

If the relation is not withion prohibited relations 

Or

if there is a custom that allows marriage in such relation

The marriage may  not be invalid.