Maybe he added the Joseph later and he was baptised just with the name William, because this does happen because my grandfather was baptised just William Flynn and he later added the name Joseph
William Costello, son of Thomas & Elizabeth (Duffy) had elder brothers named William and Joseph.
Elder William Costello birth registration Accrington 1876, mother's maiden surname Duffy.
Baptism Sacred Heart, Accrington 29th Oct. 1876, born 30th Sep. according to baptism register.
Abode Accrington.
Possible death 1887 aged 10 registered in Oswaldtwistle sub district.
Joseph Costello 2 birth registrations in 1883.
Mother's maiden surname Duffy, registered Oswaldtwistle sub district.
Mother's maiden surname Pickup, registered Accrington.
Joseph Costello baptisms at Sacred Heart, Accrington in 1883:
Parents Patrick & Elizabeth (Pickup), abode Accrington;
Parents Thomas & Elizabeth (Duffy) abode Oswaldtwistle. Baptism 19th Aug., born 28th July.
Death registration Joseph Costello 1883 aged 0, Oswaldtwistle sub district.
If this is the correct family, Joseph may have been added to William's name in memory of his other dead brother or he may have chosen it himself when he received the sacrament of confirmation.
Margaret, sister of William, married Francis Pope at St. Anne, Accrington 1911.
Are you sure it's the correct William Costello, given that the William Joseph Costello who emigrated in 1922 was apparently born in St. Helen's? (reply #5)