I don't think that Ernestine would have registered a birth twice.
I am going to look at the Ernests one at a time
Sep qtr 1890 Ernest Frederick LeFort Marylebone 1a 556
We know he was christened with mother as Ernestine no fathers name.
I am only seeing one Ernest/Ernest F bn c1890
Possibly 1901 born Surrey a visitor with the Sadd family in Dulwich
In 1909 he marries Ada - she dies in 1913.
According to Army records Ernest Frederick has children (Ada) Eileen 13/2/11 child of his first marriage and Cecil 24/6/13 (son of Victoria) and his last occupation was Publican & Motor Driver.
He married Victoria in Cambridge in 1915. He is in 1916 street directory in Cambridge at The Spread Eagle.
He appears with Victoria in 1939 Leyton area (Essex). You have already found him in that area.
Finding Ernest Saunders however could be a nightmare

I wonder if it is worth waiting to see what Marie says about her father when she marries Walter Lambert - she could be telling fibs though.