Elliptic Curve Point

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i am doing this to find out Double point on elliptic . for prime field 5 if P=(0,3) 2P=(1,3)

K.<a>=GF(p^2);K.modulus()  
R.<z>=PolynomialRing(K);

my resullt after performing point doubling is in the form of i am getting P=(3a) and 2P= (z+3a+4) which is represent (4,3) but i want (1,3) that is (z+3*a+1) how i will obtained