Hi, I have got loads of duplicate urls - the most common non-sef variation is for example:
index.php?option=com_content&id=32&view=category&Itemid=100551
index.php?option=com_content&id=32&layout=default&view=category&Itemid=100551

Would it be okay to delete one and which one should I keep?

Thanks.

Well, the problem is that you should find the source of the problem = that means a place in code / template where the incorrect link is generated. (in this case, it might be the one that is missing the layout variable) And you should fix it so it is correct (= complete in this case; that is incl. the layout variable).

Just deleting one will not help, as JoomSEF will again create it next time it will hit it in the code. (you can then only keep it in DB but "disable" by marking the correct link as active in URL manager.

Interesting, I just deleted all the layout url's today as they all had / at the end and I thought that may be an issue!
I can't find the source of the incorrect urls - some examples of these are www.africaa-z.co.za plus / and the below:
index.php/Swaziland-Suggested-Itineraries/kenya - if you load this, the site loads (eventually) but layout is incorrect and all the links are on that page are dodgy so the pages keep generating.
Your help would be greatly appreciated.

You could try turning on the URL source tracking option in JoomSEF.
Then delete the problematic URL and check the source information once it again appears in the DB.