I don't know about online dictionaries, don't use them.
Oxford English Dictionary recognizes harrassment, though it calls it obselete. Other dictionaries I have at the office give it as an alternate spelling.
It tends to be in use in the legal community with a double r. Don't know why. Washington State statutes and case law uses both, which is what got me looking into it. When the courts you're writing an appeal to use one spelling, you tend to adopt their spelling!!
If you do a search on Google, you will find it in use in both forms on various websites.