From my (basic) understanding of Chinese, I know nouns are usually connected by 是:
Noun 1 + 是 + Noun 2.
Moreover, I understand the 的 from the structure:
Adj + 的
as attaching the Adj to an omitted Noun. This omitted Noun can either be a general noun such as "ones" (红的 - the red ones) or an obvious Noun implied from the context.
I have come across the following structure for reduplication of adjectives:
Noun + Adj + Adj + 的.
Nǐ de liǎn hóng hóng de.
Your face is red.
Question: Why is there a 的 in reduplication of adjectives following structure 3?
Adj + Adj + 的should attach to an omitted noun, so structure 3 would connect two nouns (precisely, the same noun) without using 是 (structure 1). Is this 是 implied? In the example: 你的脸(是)红红的?
On the other hand, I would have expected no 的 in structure 3; in that case, we would have
Noun + Adj + Adj, which is the common simple "noun + adjective" sentences with two adjectives. In the example: 你的脸红红.
EDIT: This last point - why it is not possible to have
Noun + Adj + Adj- was addressed in https://chinese.stackexchange.com/a/36609/25265 .