Spot all (anti)diagonals with duplicated values

R, 92 86 82 78 bytes

function(m,x=row(m),y=col(m),`|`=split,`^`=Map)sum(max^table^c(m|x-y,m|x+y)>1)

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Explanation

First, we declare additional variables \$x\$ and \$y\$ that stand for row and column indices, respectively. Then we can delineate the diagonals and anti-diagonals by taking their difference and sum. E.g., for a 4x4 matrix:

\$x-y\$ gives:

0 -1 -2 -3 1 0 -1 -2 2 1 0 -1 3 2 1 0

\$x+y\$ gives:

2 3 4 5 3 4 5 6 4 5 6 7 5 6 7 8

Now split(m, x-y) and split(m, x+y) produce the actual lists of diagonals and anti-diagonals, which we join together.

Finally, we count the entries of the resulting list where duplicates are present.

Thanks for bytes saved:

-4 by CriminallyVulgar
-4 by digEmAll

J, 21 20 bytes

-1 byte thanks to Jonah!

1#.|.,&((~:&#~.)/.)]

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Explanation:

1#.                   find the sum of the  
     ,                concatenation of
       (          )   the result of the verb in the parentheses applied to
                   ]  the input
      &               and
   |.                 the reversed input
        (      )/.    for each diagonal
         ~:&#~.       check if all elements are unique and negate the result 

Japt, 31 bytes

ËcUî
ËéEÃÕc¡XéYnÃÕ mf fÊk_eZâÃl

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Explanation:

Ëc                            #Pad each row...
  Uî                          #With a number of 0s equal to the number of rows

ËéEÃÕ                         #Get the anti-diagonals:
ËéEÃ                          # Rotate each row right a number of times equal to the row's index
    Õ                         # Get the resulting columns
     c                        #Add to that...
      ¡XéYnÃÕ                 #The diagonals:
      ¡XéYnà                  # Rotate each row left a number of times equal to the row's index
            Õ                 # Get the resulting columns
              mf              #Remove the 0s from each diagonal
                 fÊ           #Remove the all-0 diagonals
                   k_   Ã     #Remove the ones where:
                     eZâ      # The list contains no duplicates
                         l    #Return the number of remaining diagonals

I also tried a version based on Kirill L.'s Haskell answer, but couldn't find a good way to "group by a function of the X and Y indices" and the alternative I found wasn't good enough.