Idiom medium 28 / 51
“one and the same”
i · The common misuse One wrong form
ii · What it meansTwo lines, no filler
The phrase is 'one and the same' — an emphatic way of saying they are identical. 'One in the same' looks plausible but doesn't parse.
iii · Memory hookKeep this one
One AND the same. The 'and' is the link that makes it emphatic.