Datacomplexity

Hello! I am new to Power BI and trying to do somethign quite complicated - or so I believe. I have two columns, one with let's say 'numbers X'. I have another column with let's say 'numbers Y'. I would like to return all X values with the same Y value for selected values in column 'numbers X', but not returning a repeat within a selected value from 'numbers X'. The issue I am running into is the lookup function is returning a repeat selected value for itself. See example below. '1' and '2' would be returned because it was found with same 'numbers Y'. '4' would not be returned because 'numbers Y' did not have a repeat value outside of itself.