Does Python forbid two similarly looking Unicode identifiers?

I was playing around with Unicode identifiers and stumbled upon this:

>>> 𝑓, x = 1, 2
>>> 𝑓, x
(1, 2)
>>> 𝑓, f = 1, 2
>>> 𝑓, f
(2, 2)

What's going on here? Why does Python replace the object referenced by 𝑓, but only sometimes? Where is that behavior described?

PEP 3131 -- Supporting Non-ASCII Identifiers says

> All identifiers are converted into the normal form NFKC while parsing; comparison of identifiers is based on NFKC.

You can use unicodedata to test the conversions:

import unicodedata

unicodedata.normalize('NFKC', '𝑓')
# f

which would indicate that '𝑓' gets converted to 'f' in parsing. Leading to the expected:

𝑓  = "Some String"
print(f)
# "Some String"

Here's a small example, just to show how horrible this "feature" is:

𝕋𝐡ᵢ𝔰_ｆ𝔢𝘢𝚝𝓊ᵣₑ_𝕤ₕ𝔬𝔲𝖑𝔡_dₑ𝕗ᵢ𝘯ｉ𝘵𝚎ℓy_𝒷𝘦_𝐚_𝚋ᵘg = 42
print(T𝗵ℹ𝚜_𝒇e𝖆𝚝𝙪ᵣe_ₛ𝔥º𝓾𝗹𝙙_𝚍e𝒇ᵢ𝒏ⁱｔᵉ𝕝𝘆_𝖻ℯ_𝔞_𝖇𝖚𝓰)
# => 42

[Try it online!](https://tio.run/##JU45DoJAFO09haW2aust7GkkaqPE0FBKCESNC5sCOia4FcZENJkYNVh4E@YCzA3wocWf92feNpIit3vdapZx4o45mYcJ3XLiREIa60DsHiYgnNijhO6YagpQ7pnqgj1jbpwsTGAoNEGCW/4SvEsaDwEU5inTbAU2646HI5Z5gCMYJ9RrFevFWqUg9TtdudTgZEmZ9gC3zuWGiGz93@6f0C4KTF2h6/B54j9vyKH1/TxsIuaGX7U1Y4NrGjsJxQdceD0disWLaRegs8kvBiZARlTOsi8 "Python 3 – Try It Online") (But please don't use it)

And as mentioned by @MarkMeyer, two identifiers might be distinct even though they look just the same ("CYRILLIC CAPITAL LETTER A" and "LATIN CAPITAL LETTER A")

А = 42
print(A)
# => NameError: name 'A' is not defined