How to get the unique data from 1 column using the duplicates from another column

Steiny88 

This is larger and somewhat more rambling approach to the problem!

What I set our to do was, first get a list of distinct numbers.  From those return a list of distinct names corresponding to each number.  Return any for which there exists more than one match.  My assessment is that it is a computationally expensive process so I am not convinced it would extend to tens of thousands of rows.  The worksheet formula is

= LET(
      distinct, UNIQUE(Number),
      matches, MAP(distinct,  MultipleMatchesλ),
      FILTER(HSTACK(distinct, matches), matches<>"")
   )

with a Lambda function MultipleMatchesλ

= LAMBDA(num,
     LET(
         matchedNames, UNIQUE(FILTER(Name, Number=num)),
         multiple?, COUNTA(matchedNames)>1,
         IF(multiple?, TEXTJOIN(", ",,matchedNames),"")
     )
  )

Steiny88 

That could be conditional formatting rule with formula

=COUNTIFS($B$2:$B$10,$B2,$A$2:$A$10,$A2) <> 1