Let me try.
1. The answer to your first question is that it only matters if you're trying to figure out how to handle it programmaticaly. In this case you might have to ask the developer if this is being done by a session id. To me it looks more like a URL parameter, but without a live example I wouldnt know, could you provide the website in question? If not try visiting the website once, clear your cache and then visit again and see if the number after "return_id" changes. if it changes that is a session id. If it stays the same have a friend visit the website in the same manor and see if the number stays the same, if it changes then there's a good chance that this is a session id.
No matter if it's a session id adding it or not "return_id" is technically a URL parameter that is triggered by a session id.
2. The second question is still a bit vague, so let me see if this is correct. are you asking how to treat the duplicate content once you know what is causing it? If so, then follow these rules.
If the content changes significantly in the presence of the session id or parameter then this is not duplicate content. If the content does change do the following:
- make sure to use rel canonical for the root URL. In your example that would be: www.example.com/index.php?mact=Calendar
- set the URL parameters in Google and Bings webmaster tools to treat the parameter correctly.
- When the parameter or session id is present add the noindex, follow robots tag. this will allow the bots to spider through and pass on link juice in the event that someone links to your parameter versions
I think you have a larger issue, which is that your website's code is using the index.php to generate all of the pages, in the example that is calendar. This is a common mistake that programmers make since they work to do things as quickly and efficiently as possible. Its far easier to keep all of the code in the one file than to create several different dynamic files that work with each other.
If you dont have the ability to break this down and generate out different pages you might be able to use URL Rewrites to make browsers and bots think the URLs are actually different.
