Bit of a general question, although I'll supply my specific details: is there any reason why one person would appear on two different electoral rolls for the same year?
Specifics: Robert John McMartin and his wife Annie Christina McMartin (nee O'Neill) appear at two different addresses in the 1930 electoral roll.
New South Wales > 1930 > Cook > Alexandria > M > 3 shows them both at 288 Belmont St., Alexandria.
New South Wales > 1930 > Parkes > Earlwood > M > 1 shows them both at 15 Potter Ave., Canterbury.
Seems rather unlikely that there would be two McMartin couples with identical first and middle names.