Perhaps there are legitimate reason to consider "last digit" and "modular arithmetic" to be treated not as exact duplicates.
Already somebody who knows how to multiply can notice and understand how last digit behaves when computing powers or multiplying numbers.
So question of this type can be asked by somebody who knows nothing about congruences.
Basically I am saying that noticing some staff about last digit of products or powers needs lower level of abstraction than doing the same thing modulo arbitrary number.
From higher point of view, both are the same thing. But I can hypothetically imagine somebody (probably young enough) who would be able to understand answer showing how the last digit repeats when computing powers, but who would have problems grasping what congruence is.
@Jyrki in case you happen to stumble upon this, it was partially addressed to you. However, it is certainly not a big deal.
I think that my interpretation above agrees with the standpoint I explained in my answer to your question about exact duplicates (partially motivated aby $a^b\bmod c)$:
The OP asked in original version (now substantially rewritten a further from a rant): "Am I the only one concerned about duplicates?" My answer to this would be certainly not, as witnessed by many people suggesting duplicates, voting to close as duplicates, discussions in chat about duplicates (...
So perhaps it would be a reasonable idea to have a good thread to serve as a canonical duplicate target for questions asking about finding last digit (or last two digits) of some power $a^b$ written is a decimal number, similarly as How do I compute $a^b\,\bmod c$ by hand? serves for modular arithmetic.
Of course, there is an overlap - the question would (or even should) have answer using congruences modulo 10. But it also should have answer which does not use symbol for congruences and is accessible to people not knowing anything about divisibility and congruences. (At least if the argument why the two questions are not considered the same is the one I described above.)
But I am not sure whether somebody is willing to spend some time by looking for a question of this type which has good answers and would be good duplicate target.
Maybe reasonable place to look could be among questions which are often linked?
