reproduced with go1.15, but I suspect it's always been that way since -benchtime was introduced.

Sample code:
https://play.golang.org/p/It6iFOHZldA

So, I wouldn't have noticed this, but I have a benchmark for a pathological case, and the thing about the pathological case is it only shows up with reasonably large inputs -- but shows up by exploding runtime into minutes. So, of course, I run with -benchtime 1x.

... And it runs twice.

Looking at the "got here" output, it's pretty obvious why:

N:1, previousN:0, previousDuration:0
=>
N:1, previousN:1, previousDuration:85948

The thing is, when I've explicitly specified a number of times, I probably don't actually need the initial calibrating run.

There's a reasonable workaround (make it a "Test" function which does its own timing), but it'd be really handy to have all those convenient benchmark things, like reporting memory usage. I thought of trying to use testing.Benchmark(), but it does the same thing -- it runs the function twice.

The best alternative I can think of is to make the exploding parameter a b.N value, and run with -benchtime 1000000x or something like that, but this is completely incompatible with the way any other test/benchmark would want to run, or all the very thoughtful guidance not to vary quadratically on b.N.