Hi,
Referrals from doubleclick.net are clicks on your AdWords ads showing on the Google Display Network specifically, ads showing on publisher sites in the AdSense program for which the destination URLs have not been tagged.
If you notice doubleclick.net appearing in your reports, check to see if auto-tagging is turned on in your AdWords account, and that your own campaign variables are labeled correctly.
googlesyndication is where the clicks from Google AdSense ads are directed to before you go to the advertisers website to count the click on the ad. Referrals from googlesyndication.com are clicks on your DoubleClick ads.
Hope I am able to answer your query.
Regards,
TM