one and the same
Identical, as in Gloria's grandfather had been, at one and the same time, a physician at the court and a general in the army. This expression is an emphatic form of “the same.” [Mid-1800s]
Why Do “Flammable” And “Inflammable” Mean The Same Thing?Why are these two words so confusing? Well, flammable and inflammable both mean “able to be set on fire.” Strangely, the "in" prefix in inflammable doesn't make the word mean "not flammable," as you might think.
Were P And R Once The Same Letter?Do you ever stop and look at the shape of our alphabet? Each letter looks natural to us now, but all those lines and circles have unique histories. It’s easy to make assumptions that our letters make sense, that they developed in some orderly logical way, and one reasonable assumption would be that P and R are related to each other based on their form. …
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.