The coefficients \( a,b,c \) in the quadratic equation \( ax^2 + bx + c = 0 \) are chosen from the set \( \{1,2,3,4,5,6,7,8\} \). The probability of this equation having repeated roots is:
\( \dfrac{3}{256} \)
\( \dfrac{1}{64} \)
\( \dfrac{1}{128} \)
\( \dfrac{3}{128} \)