time complexity of `pandas.DataFrame.index.is_unique` looks to be constant for unique index and linear for non-unique index

is_unique_time_complexity.py

from timeit import timeit

import matplotlib.pyplot as plt
import pandas as pd

df = pd.DataFrame(range(10), columns=['A'])

times = []
lengths = []
for i in range(26):
    # try changing ignore_index param to False and compare the result
    df = pd.concat([df, df], ignore_index=True)
    times.append(timeit('df.index.is_unique', number=1, globals=globals()))
    lengths.append(len(df))

plot = pd.DataFrame(times, index=lengths).plot()
plt.show()

Typical shape of the plot when ignore_index is True:

Typical shape of the plot when ignore_index is False: