Of course they don't. An infinitesimal is defined as being larger than 0, but smaller than any real non-zero number.
So... if 1 and 0.999... both represent real numbers, the difference between them can't be one of those infinitesimals, because they aren't real numbers, and the difference of any two reals is another real. (The real numbers are closed under addition and every real has an additive inverse, so they are closed under subtraction as well).
The difference between those two numbers is therefore zero or non-zero.
If the difference is non-zero we reach the contradiction that it's smaller than itself, which is impossible for a real number.
There are only two possible conclusions:
1) 0.999... doesn't represent a real number.
2) It represents the same real number as 1.
Now, OK, the assertion that 0.999... represents a real number at all does depend on a limiting process. The real numbers are complete by definition: any convergent sequence of real numbers has a real limit. The sequence 0.9+0.09+0.009+... therefore has a real limit and that limit is what we mean by 0.999...
But once you accept that 0.999... is allowed to represent a real number, 1 is the only possible value it can have. Anything else will lead to a contradiction.