I use UTM parameters to track all the links I use on different advertising channels (Adwords, FB ads, Bing ads, etc.).
However, since UTM parameters will cache when a user first comes to my website if that user initially came from a Facebook Ad, then left my site and came back through a Google Adwords Ad the UTM parameter will list Facebook Ad but the conversion actually happened with the Adwords ad.
So my question is how can I track if Facebook ads is actually helping/complementing my Adwords ads?
What are you currently using to track conversions? Google Analytics? There are a number of ways to accomplish what you are speaking about and it will depend on how robust of a solution you desire and also what your sales cycle looks like.
One simple solution is to use the Google Analytics "Multi-Channel Funnels - Top Conversion Path" feature to track source and medium of each touchpoint throughout the conversion process. Select "source/medium path" as your primary dimension and then "campaign path" as your secondary dimension. This should give you a basic Idea of how people are engaging with your site prior to a conversion. You can also use the Attribution Model Comparison tool to discover which channels have the most impact at certain points during the buying cycle.
If you are in a more complex situation (IE, b2b lead gen with a sales team and long sales cycle) there are more robust solutions such as creating a UUID that can push to your CRM and back into Analytics upon sales cycle completion to give a more clear idea as to which channels, campaigns, keywords etc are resulting in actual sales vs which ones created leads.
Hope that helps. Let me know if you want a have a quick call to discuss further.
As Ross mentioned, you can get started with some pretty simple attribution modelling out of GA, rather than using the platforms themselves to report conversions individually and hence getting duplicates.
Important thing to keep in mind with all attribution modelling is that you're trying to best account for contribution along the path to conversion, it's never going to be 100%, but you're just trying to become "less wrong".
Facebook's built-in conversion tracking does a really good job of sorting these things out. They run some complicated scripts to determine whether their ads was the origination of a conversion.
However, if you really want to do your own segmenting I'd work out a system to send users to a unique conversion page based on where they entered the site. It's actually pretty easy and and would give you a definite answer as to where your best / lowest-cost conversions come from.