I have not seen this referenced anywhere as such, but I can think of no way to construct counter-examples, either.

The answer is very simple, actually—it does not even have anything to do with repetition or relative pronouns at all:

**A complement clause without a subject must not be overtly marked.**

To explain more fully:

A _complement clause_ is a subtype of [noun clauses](http://en.wikipedia.org/wiki/Noun_clause#Noun_clause), themselves a type of subordinate clauses. Complement clauses begin with the subordinate marker ‘that’ and function as the object of a variety of verbs, mostly verbs that indicate feeling or reporting verbs.

The subordinate marker is nearly always optional in English:

> I think that my husband is having an affair.  
I think my husband is having an affair.

This is a lot like in [relative clauses](http://en.wikipedia.org/wiki/Relative_clause), where the relative marker (pronoun) _that/who(m)/which_ is optional as long as it is not the subject of the clause:

> The man _who_ is walking on the street. [_who_ = subject; not optional]  
The man _[who(m)]_ I see walking on the street. [_who(m)_ = object; optional]

The rule of when the marker is optional is a bit different in the complement clauses, though: there the rule is that if the clause _has no overt subject_ (meaning that the actual subject of the clause is located outside the clause, in the main clause that the complement clause is embedded in), then the marker is not optional, but actually _disallowed_. Compare the following sentences where I also show that the presence or absence of another _that_ (the relative marker) is not relevant. The relative markers are in **bold**, while the subordinate markers are in _italics_.

### With relative marker ###
> The new car **that** I hope _(that)_ my cheating husband will buy me.  
My cheating husband **that/who(m)** I hope _†that_ will buy me a new car.

### Without relative marker ###
> The new car I hope _(that)_ my cheating husband will buy me.  
My cheating husband I hope _†that_ will buy me a new car.

In the first sentences here, the complement clause is (basically) _my husband will buy_, which has a subject and a verb. In the second sentences, the clause is _will buy_, which has no subject.

Since there is no subject, overtly marking the complement clause with the subordinate marker _that_ is impossible.

Note that the clause does of course have a logical subject, but it is not located in the clause itself. It is, in fact, the relative marker _that/who(m)_. This leads to problems if using _who(m)_, because the poor pronoun ends up being the object in the relative clause (_whom I hope_), but the subject in the embedded complement clause (_who will buy_)—and it becomes pretty much impossible to decide whether to use _who_ or _whom_. This is a case where English grammar simply collapses and there is no ‘right’ or ‘wrong’.