How can this happen?

Someone on my ignore list has whispered me twice in the last few days. I should not receive their messages if I have ignored them.

Since I have two accounts I was able to test this and the results are as follows (and as expected):

Player A ignores Player B
(I tested this with Player B being online, away, DND and offline status.)
-Player A is in Player B's friends list, but Player B cannot see when player A is online. The only way Player B knows that Player A is online is to try to send a message to the (active) character.
-Player B messages player A and receives the message "Player A is ignoring you." Player A does not actually receive any message from Player B and has no idea Player B is attempting to message them because they have ignored Player B. Otherwise, if Player A is not logged in the message "Player A is offline" is received by Player B.
-Player A can message Player B and Player B will receive the message but can't respond.

As I said, this person is in my ignore list and I have confirmed that their name is correctly spelled - yes, I was ignoring the right person! So, how is it possible that this person was able to whisper me and that I would actually receive their message?

The only other explanation I can think of is that this person could have made a character on a second account that was so similarly named to the one I have ignored that I did not notice because I was busy.

Yep, you answered your own question, only possible explanation

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Now playing: Guns N' Roses - Sweet Child O' Mine
via FoxyTunes

The moral of the story: It is best to be more clever than the person you wish to ignore.