There are two "styles" of filter head and banjo bolt.
The "Early" version uses one o-ring and a copper crush washer.
The "Later" version uses two o-rings, no crush washer.
Here is a picture I took of my "Later" style banjo bolt on my '85 300D. Notice that it has two obvious spots for o-rings.
If this is the style on your car and it's not sealing well you're simply using the wrong size o-rings.
You can try buying them directly from Mercedes or from Pelican Parts using the part numbers, that will get you the exact o-ring that Mercedes intended to be used in this spot.
I can't find a good picture of the early style online. I'll have to take a pic from my '79 next time I do the filter...
However it only has a spot for one o-ring, the lower one. The top of the bolt it sealed with the crush washer. Indeed you do have to tighten it pretty tight to get it to seal, since you have to crush said washer to get a good seal. Once crushed though it seals fine.
If you have the early style after all and find it to be frustrating to get a good seal you can always swap in the late style filter head off a junkyard car.