Where is the verb in reduplication of adjectives "Noun + Adj + Adj + 的"?
EDIT:
blackgreen's answer reminded me that the noun in “Noun + Adj + Adj + 的” is not the subject of a [SVO] sentence but the topic of a [topic + comment] sentence.
There are two ways to interpret which is the topic in 他脸红红的

[他(的)脸 (topic)] [红红的 (comment)] = [his face (topic)] [is very red (comment)]

[他 (topic)] [脸红红的 (comment)] = [he (topic)] [is red-faced (comment)]


Either way, 的 is acting as an adjective marker that marks 红红 as an adjective. and the 是 (is) between the topic and the comment is omitted
As for the rest of the explanation, I cannot explain it better than blackgreen did, so I am not going to try.
Different example:

[關羽的脸 (topic)][(是)红红的 (comment)] = [Guan Yu's face (topic)][(is) very red (comment)]


[關羽 (topic)][脸红红的 (comment)] = [Guan Yu (topic)][(is) red-faced (comment)]

Example of using 红红的 in SVO sentences:

[關羽][有] [一張红红的脸]
[Guan Yu (S)] [has (V)] [a very red face (O)]