Why are passwords with repeating substrings weak?

Question

Many websites have password strength checking tool, which tells you how strong your password is

Lets say I have

st4cK0v3rFl0W

which is always considered super strong, but when I do

st4cK0v3rFl0Wst4cK0v3rFl0W

it is suddenly super weak. I've also heard that when password have just small repeating sequence, it is much weaker.

But how possibly can the second one be weaker than the first one, when it is twice as long?

Answer 1

Sounds like the password strength checker is flawed. It's not a big issue, I suppose, but a repeated strong password is not weaker than the original password.