Do similar spells stack ?

My necro has Reapers Mark (Elite) at -4 degeneration, Suffering -2 and Faintheartedness -2. Will these stack giving -8 degeneration ?
Thanks

yes they will, but if u where to cast repers mark again it will jsut renew it rahter than add an aditional 4 degen hope it helps

Yes. Degen from different spells or conditions stack. As Olly said though, recasting the spell on somebody will merely reset the timer on the length of the duration, not increase it.
Also note that there is a maximum on degen/regen of -10 or +10 at all times.

Keep in mind that while the maximum net is 10 pips +/-, the gross can be significantly higher an effect the benifits of healing on your target. As an example: Say you hit a target with bleeding, poison, disease and Life Siphon all at the same time for a total of -15 pips. The maximum degen on their health bar will only be 10, but there are 5 "in the bank"...If the target is a ranger with high Wilderness Survival is using Troll Ungent at +10 regen, they will then only counter the first -10 and still suffer the remaining -5 "banked" pips...Causing more than -10 pips of gross degen is desirable if you want to keep pressure on your target, so feel free to overdo it. Hexes and conditions both make it harder for targets to shed the degen as removals tend to work on one or the other...