Use a rule with an expression more than once

a + x ^ 2 + y ^ 6 /. {x -> 2 + a, a -> 1}

1 + (2 + a)^2 + y^6

a + x ^ 2 + y ^ 6 //. {x -> 2 + a, a -> 1}

10 + y^6

log[a b c d] /. log[x_ y_] -> log[x] + log[y]

log[a*b*c*d]

log[a b c d] //. log[x_ y_] -> log[x] + log[y]

log[a*b*c*d]

ReplaceRepeated[x, x -> x + 1, MaxIterations -> 1000]

1000 + x

ReplaceList[log[a b c], log[x_ y_] -> log[x] + log[y]]//Column

Column[{}]