Off topic:Count frequency of elements of one file in another file

Question

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I have 2 file

file 1-

M1  M2
M1  M3
M3  M1
M3  M2
M4  M5
M7  M6

file 2-
P1  M1,M2,M5,M6
P2  M1,M2,M3,M5
P3  M4,M5,M7,M6

I want to count the frequency of the file 1 pair of column 1 and column 2 in file 2. Expected output-

M1  M2  2
M1  M3  1
M3  M1  1
M3  M2  1
M4  M5  1
M7  M6  1