Brihat Parashara Hora Shastra
मातृकारकमेवाऽन्ये वदन्ति सुतकारकम् । द्वौ ग्रहौ भागतुल्यौ चेज्जायेतां यस्य जन्मनि ॥ १६॥
If two Grahas have the same longitude, both become the same Karak, in which case there will be a deficit of one Karak.
english translation
mAtRkArakamevA'nye vadanti sutakArakam । dvau grahau bhAgatulyau cejjAyetAM yasya janmani ॥ 16॥
hk transliteration by Sanscript