When we calculate electric potential energy of a two particle system, say first I bring +q1 and then I bring +q2 against q1's electric field. Say I get tha q2 charge to a point r distance away from q1. Now potential V1 at that point( where q2 resides) due q1 is kq1/r^2 and hence the potential energy of q2 is = q2(V1)= kq1q2/r .Similarly at point where q1 resides which is again r distance away from q2, potential V 2 at that point due to q2 is kq2/r^2. And hence the potential energy of q1 becomes q1(V2)= kq1q2/r. So the potential energy of both the charges are same and hence when we talk about the potential energy of the system, it should be 2(kq1q2/r). But in books it's written that the potential energy of a two particle system is kq1q2/r ( why not it's twice?) I agree that the time when only q1 charge arrived( no other charge present) potential energy of be q1 charge at that time will be zero but after the arrival of q2 charge in the presence of q1 , then off course potential of q2 exist but at the same time potential of the q1 charge will also exist as q2 has its potential created at the point where q1 resides and hence q1 would also have potential energy. And hence potential energy of the system should be the potential energy of q1 and q2 and it is 2(kq1q2/r), but why in books it's written as kq1q2/r?

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When we calculate electric potential energy of a two particle system, say first I bring $+q_1$ and then I bring $+q_2$ against $q_1$'s electric field. Say I get that $q_2$ charge to a point $r$ distance away from $q_1$. Now potential $V_1$ at that point  (where $q_2$ resides) due $q_1$ is $kq_1/r^2$ and hence the potential energy of $q_2$ is $q_2V_1= kq_1q_2/r$.

Similarly, at the point where $q_1$ resides, which is again $r$ distance away from $q_2$, the potential $V_2$ at that point due to $q_2$ is $kq_2/r^2$. Hence the potential energy of $q_1$ becomes $q_1V_2= kq_1q_2/r$. 

So, the potential energy of both the charges are same and hence when we talk about the potential energy of the system, it should be $2(kq_1q_2/r)$. But in books it's written that the potential energy of a two particle system is $kq_1q_2/r$. Why is it not twice that? 

I agree that the time when only charge $q_1$ arrived  (with no other charge present) the potential energy of charge $q_1$ at that time will be zero but after the arrival of charge $q_2$ in the presence of $q_2$, then of course the potential of $q_2$ exists but at the same time potential of the charge $q_1$ will also exist as $q_2$ has its potential created at the point where $q_1$ resides, and hence $q_1$ would also have potential energy. And hence potential energy of the system should be the potential energy of $q_1$ and $q_2$ and it is $2(kq_1q_2/r)$, but why do books claim it is $kq_1q_2/r$?