Why is my program calculating the compound interest twice

I want to calculate the time it takes for the compound interest to reach a certain goal by using Pn=(1+i)Pn−1.
What I have done is:

def my_saving_plan(P0,i,goal):
    P = [P0]
    years = 0
    for n in range(0,10000):
        if P[n] < goal: 
            P.append((1+i)*P[n-1])
            years += 1
            print(P)
        elif P[n] >= goal:
            return years
print(my_saving_plan(1000, 0.07, 2000))

But What I get from that is the number of compound interest twice. So for this example what I get is:

    [1000, 1070.0, 1070.0, 1144.9, 1144.9, 1225.0430000000001, 
1225.0430000000001, 1310.7960100000003, 1310.7960100000003, 
1402.5517307000005, 1402.5517307000005, 1500.7303518490005, 
1500.7303518490005, 1605.7814764784307, 1605.7814764784307,
 1718.186179831921, 1718.186179831921, 1838.4592124201556, 
1838.4592124201556, 1967.1513572895667, 1967.1513572895667, 
2104.8519522998363]
    21

Also I was wondering if there is a way to use a while loop instead of putting a ridiculous number in the for loop.

>Solution :

Might as well turn my comment into an answer:

def my_saving_plan(P0,i,goal):
    P = [P0]
    while P[-1] < goal:
        P.append((1+i)*P[-1])
        print(P)
    return len(P) - 1

print(my_saving_plan(1000, 0.07, 2000))

From my comment:
You have an off-by-one error in your append line. n is already the last element, and using n-1 will be one behind that, thus causing duplication.

You could also use while P[-1] < goal instead of for. Index -1 always refers to the last element. You don’t need n or years at all.