Thanks for you responses.  I just managed to fix the problem (we are running dnn 4.8.2).   The problem was caused by one of two things. I'm not sure which since I changed them both and then recycled the app pool and the problem was fixed so either one could have fixed the problem

1.  Multiple dnn installs - we had an old dnn install running and was attached to the same app pool as our production dnn install. 

2.  Mutliple sites in IIS pointing to same dnn install - we had 2 sites pointing to the same dnn install.  We removed the second site from iis.

In the process we isolated the dnn install to it's own app pool and the recycled.

After this our scheduler stopped firing multiple times.  Anyway, like I said I'm not sure which item fixed it but it's working great now.

Thanks for your help.

if the old installation was still on DNN 3 and ASP 1.1, this could be the reason - never mix ASP.Net 1.1 and 2.0 applications in a single pool.

that's another no go, you can easily assign multiple hostheaders to the same IIS web site for multiportal installations.