The purpose of a SHA-256 hash is to detect corruption of the data. It is a checksum. If you don't agree, then what is its purpose?
you could even require a valid seed phrase to be 46 words long by just duplicating the original 23 word phrase. then the last 23 words would be your checksum for the first 23 words. and they could detect AND fix errors. 
You are contradicting yourself. Your 46-word phrase would detect an error, but it could not fix it because if the duplicates don't match, you don't know which one is wrong. So, it is no better than the BIP-39 checksum.
For example, lets say the seed phrase is hotel obvious agent lecture gadget evil jealous keen fragile before damp clarify
Now what I do is write it twice:
hotel obvious agent lecture gadget evil jealous keen fragile before damp clarify
hotel obvious agent lecture gadget evil jealous keen fragile before damp clarify
I then compare them. I see they are the same. no mistakes were made. we know that with 100% certainty. No need for any checksum. The eyes are good enough. Now lets say I was not paying attention and wrote it down like this:
hotel obvious agent lecture gadget evil jealous keen fragile before damp clarify
hotel obvious agent lecture gadget jealous before fragile damp clarify
I made some serious errors but its very easy to compare and fix. Try that with sha256 and see how long it takes you, if you can even fix it at all.