Re: Paying zakat twice
USResident: Brother, it is wrong assumption that Zakat is to purify your wealth. Zakat is what Allah has made obligatory on Muslims who believe, and that is nothing to do with purifying anything, it is to do with obeying command of Allah.
Now, apart of Allah sending Quran, Allah sent prophet (SAW) to teach Islam. It was duty of Prophet (SAW) that Prophet (SAW) teaches everything about Islam to Muslims around him. Every thing that was obligatory on Muslims from Allah, Prophet (SAW) taught all Muslims of his time, and did that if he was in the situation to do that.
Thus, Islam consists of two things. One is Quran that we all can read. Another is what Prophet (SAW) showed or told Muslims to practice (Sunnah). All Muslims during the time of Prophet (SAW) knew that and they told all new Muslims and their children. So Sunnah is coming down to Muslims, generation to generation, from Prophet (SAW) to all Muslims at present, without any narrators in between or any doubt of its being distorted.
We all learn (see, hear and practice) this Sunnah of Prophet (SAW) as we grow, from elders around us, who learned in same way when they were young from their elders around them, who learned same way as our elders did, and so on until we reach the time when Muslims learned that from Prophet (SAW). New Muslims (converts or some like to call reverts) learn Sunnah from other Muslims around them. So, all Sunnah is preserved amongst Muslims in this way (within the body of Ummah), and when we see any variations, the reason is that they are not strict part of Sunnah or command of Allah, and thus Prophet (SAW) use to vary them in his actions too, and those that followed took what they felt as most suitable for them, but later Muslims made them obligatory and now fight with each other on that.
Now coming to your question, we learn that Propjet (SAW) told Muslims to give Zakat every year on savings that stayed with them for a year and on goods they held for one year (including property) that are not in their use, or any other wealth held for one year that they claim to be theirs (like loan to others). From Prophet (SAW) we also know that 2.5 percent is value of Zakat on the value of most things.
Anyhow, since 2.5 percent Zakat is to be given on all saving held for one year, every year, and this teaching of Prophet (SAW) effected many Muslims during the time of Prophet (SAW), all Muslims even today knows this ruling without doubt, coming to us from generation to generation. So, there is no doubt on this issue, that Zakat should be given on all savings that one hold for one complete year, regardless of Zakat is given on same savings in previous years or not.