Maps do not support duplicate keys, so HashMap also does not support duplicate keys. So "it supports one duplicate key" is wrong.
"It supports null" means that you can use null as a key or value. Hashtable does not support null for keys or values (you'll get a NullPointerException if you try to use null as a key or value in a Hashtable).
I would like to know what are the similarities between ArrayList and HashMap, I was asked in an interview this questions. I mean I read somewhere that both can be used for faster iteration and random access. I would like to know if there are more similarities out there between ArrayList and HashMap.