Is there a German feminine noun whose plural form is itself?
Example (of the noun I am after, but being feminine)
der Löffel / die Löffel
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One candidate is Mark (as a currency):
Dafür bezahlte man fünf Mark.
However, it could be argued that this is not really plural, but a unit-of-measure singular (cf. zehn Grad, zwanzig Stück).
Another candidate is the acronym SMS, which in Germany (not in Austria or Switzerland) is feminine:
Ich habe zwei SMS von ihm bekommen.
There are two regular feminine words that form a plural without ending, but the plural form has umlaut: Mutter – Mütter, Tochter – Töchter.
Normally, null plural is limited to masculine and neutral nouns.
All the answers ATOW miss the point that for a "word" to have the same "wordforms" they must be the same in singular and plural in the corresponding declination case, not only for the nominative case, i.e. sing.nom. = pl.nom., sing.akk. = pl.akk., sing.dat. = pl.dat., and sing.gen. = pl.gen.
AFIK all pl.dat. forms end with an "n" wich is always different form the sing.dat form.
In your example LÖFFEL you have der Löffel [sing.dat.] vs. den Löffeln [pl.dat.]