Which version of Excel are you using? If you have Excel 2010
or later then there is a specific function for this, RANK.AVG
, e.g. if your data is in A2:A30
use this formula in B2 copied down to rank as required
=RANK.AVG(A2,A$2:A$30,1)
In earlier versions of Excel you can use this formula to give you the same results
=RANK(A2,A$2:A$30,1)+(COUNTIF(A$2:A$30,A2)-1)/2