“One in the same” will generally be considered wrong. No dictionary recognizes it. You should avoid it and use the standard form of “one and the same.”
The verbal noun verbing is first cited a surprisingly long time ago, in 1766. But, as the OED notes, it was rare before the late 20th century...once a feature of language is visibly or vocally shunned, fatwas against it take on a life of their own.
Even more exciting was to discover that to regale is a loanword…from Spanish. At least, it is according to the third edition OED, which presents convincing evidence in support. Other dictionaries give a French origin.