How to properly split train and test to measure performances?
welcome to the forum. I dont know what you exactly mean with 1) „common samples“. 

But train and test should have no common rows/observations. They should be two distinct sets. 

Having duplicates in test is not a good idea since you want to test how well your model works on data not seen so far by the model. Duplicates in test shrink the number of unique test cases. So there is little use of duplicates in the test set.