Pose, I know exactly what you mean. Some people goes through their life with open hands always awaiting that someone is willing to help them. They are so familiar with this manner that they don't see any other way. They also could need your help but in another form. To refuse help is sometimes the only way to be a real helping hand!
But unfortunality in the most cases you will figure it out after you've been hurt by your so called 'friend'...
To be careful is not the worst thing because you'll need your life energy for your own and for your real friends. And, maybe the man for whom you have stopped giving help had learnt to help himself in the meantime and is now on his way to get also a real friend of yours. Okay, mostly they're not but don't give up the believing because sometimes they really are...
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Greetings from Frankfurt (Germany),
Sheriff ;-)