Source Code:

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surface_n = int(input())
x_land = []
y_land = []
for i in range(surface_n):

    land_x, land_y = [int(j) for j in input().split()]
    x_land.append(land_x)
    y_land.append(land_y)

data = zip(x_land, y_land)

for b in data:
    df = pd.DataFrame(data, columns =['x', 'y'])
	duplicateDF = df[df.duplicated(subset='y', keep=False)]
	print(duplicateDF, file=sys.stderr, flush=True)

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Test 3 Input / Output:
Input:
      x     y
0  1000   500
1  1500  1500
2  3000  1000
3  4000   150
4  5500   150
5  6999   800

Output:
      x    y
3  4000  150
4  5500  150

Test 4 Input / Output:

Input:
       x     y
0    300  1500
1    350  1400
2    500  2000
3    800  1800
4   1000  2500
5   1200  2100
6   1500  2400
7   2000  1000
8   2200   500
9   2500   100
10  2900   800
11  3000   500
12  3200  1000
13  3500  2000
14  3800   800
15  4000   200
16  5000   200
17  5500  1500
18  6999  2800

Output:
       x     y
0    300  1500
2    500  2000
7   2000  1000
8   2200   500
10  2900   800
11  3000   500
12  3200  1000
13  3500  2000
14  3800   800
15  4000   200
16  5000   200
17  5500  1500