Being basically cheap, the book I’ve been (re)reading on my smart phone lately is Agatha Christie’s The Secret Adversary. A major plotline relies on a woman’s name – Jane Finn.
Why is this considered such an odd name? It seems fairly normal to me. Is this a post-WWI thing? Or maybe it’s odd in Great Britain and not here? (I remember being confused about why people in another Christie book remarked that a brother and sister were Pip and Emma; it was years later before I figured it out.)
A name does not have to be particularly odd for it to be a remarkable coincidence to hear strangers mentioning it when you have just recently come across it yourself, in a different context.
All I can think of is that “Mickey Finn” has connotations here in the US. (More so long ago)
I didn’t read any connotations of oddity in the question (but I have not read the book). He just seems to be asking “I heard some guys talking about a ‘Jane Finn.’ Do you know who that is?”
It depends on how you interpret, “Did you ever hear such a name?”
I hear it as, “Such a name! It’s so strange!”
Others obviously think it means, “That particular name.”
Yes, that’s all it is. It’s not a comment on the name being odd, just a slightly old-fashioned turn of phrase. “Did you ever hear such a name?” just means “Have you ever heard that name?”
It’s similar in phrasing to “Did you ever hear such a thing?” which usually implies an unusual event. I can see the confusion.
Can you save me a few years and tell me now? 
“Pip Emma” was RAF slang for “p.m.”, as in times. Ack Emma was a.m.
You have to keep in mind that this was written 90 years ago, and the language to modern ears is a little funky. I see three other remarkable indicators in just that tiny bit you quoted:
“Funny scraps one does overhear” - this just sounds old-fashioned. I think it’s the “one does overhear” as opposed to “you overhear.”
“to-day” - old spelling of “today.”
“some one” - not “someone”?
That should give you a clue that you may have to interpret “Did you ever hear such a name?” in a way that you’re not accustomed to.
What is being asked is, “Did you ever hear of her?”
Tommy is wondering if Jane Finn was some sort of celebrity that he hadn’t heard of. In addition, when he heard the same name said by two different people in different contexts, that struck him as a strange coincidence. The name only has to be a little out of the ordinary: “John Smith” would have been too common to notice.
I guess I was a bit too concise in my quoting.
Tommy and Tuppence have run into each other on the street. The last time they saw each other was during the war. They go to tea:
This is the first time the name Jane Finn is mentioned in the book. I would have thought that if Tommy wanted to mention how names come up he would have said something like, “Wonder who Mabel Lewis is?”
It just seems to me there has to be something unusual about the name Jane Finn, or why else would Tommy have brought it up?
Of course maybe it’s just poor plotting on Christie’s part. In Chapter 2 Tuppence meet with a man, decides to give him a false name and, having forgotten the conversation with Tommy, says she’s Jane Finn, thinking she made it up. The man she’s meeting with is shocked and thinks she’s there to blackmail him. In what is a very clunky coincidence, the man is one of the two Johnnies that Tommy had overheard. The whole Jann Finn mystery is much of the plot.
But it would have been a lot simpler if Christie had used the name Jane Finn in one of the overheard conversations at the tea shop. Which is what leads me to believe there must be something about the name that is odd enough for Tommy to have mentioned it.
The brother and sister in question were twins, born just after 12 noon, and their mother joked she was going to call them “Pip” and “Emma”. The novel is A Murder Is Announced.
Agatha Christie evidently thought the name was odd. From here:
Well, according to How Many of Me?, there are currently only 30 people in the US named Jane Finn. 23,617 people have the surname Finn; 393,883 people are named Jane; but only 30 Jane Finns.
Contrast this with 3,470 people named Jane Smith; 2,015 named Jane Brown; 430 named Jane Rogers; and, surprisingly, 17 named Jane Doe.
I have no idea what all that tells you about this. Frankly, I just wanted an excuse to go play at that site. 