Muqattal is a passive participle on the pattern mufaʿʿal, associated with Form II qattala from the root q-t-l. The intensified formation suits the poem’s notion of a heart repeatedly tormented.
In qalbin muqattalin, the heart is the noun modified by muqattal; both are indefinite and genitive. The adjective tells us that the heart targeted by the two arrows is already badly wounded.
ExpressionMorphological or literary distinction
QātilīActive participle: causing injury
MuqattalForm II passive participle: repeatedly wounded