"Redundant" involves repetition. In the following example, there are two examples of redundancy: "This blue, azure shirt is torn and ripped." Blue and azure are redundant, and torn and ripped are redundant. Note that these redundancy pairs do not include words that are exactly synonymous, but which are close enough in meaning that one would usually consider them redundant. Two points here: 1. Neither word in such a pair is necessarily the redundant one; either one can be considered redundant, depending on which one you consider to be the more important, useful, or accurate one in the given context. Commonly, the second word is considered the redundant one, but that is merely because the first word got a chance to establish itself before the second one came along; if you were revising the text, you might choose to keep the second, not the first. 2. The same word repeated ("this blue blue shirt") is an example of redundancy, but this is usually done for emphasis, or for poetic effect, and so is seldom saddled with the accusation of redundancy. Thus, "redundant" does tend to carry the implication of an unnecessary repetition.
A tip: To help you remember this, note that "redundant" begins with "re," as in "repetition." That piece of these words means "again."
"Superfluous," on the other hand, refers to something that is more than what is necessary. Think of water running over the rim of a glass when you continue to pour water into it beyond its capacity. The water over- (super) flows (fluous). Often something superfluous is so because it is needlessly repetitive, and this confuses the picture a bit. But in my opinion, "superfluous" is better used when the element is not repetitive, but is genuinely not needed, as in this example: "After George embedded the fence post in thirty pounds of concrete buried underground, the brick he balanced atop the post to hold it down was superfluous."
I would not consider it redundant to have if...then...
Leaving aside the point of using then to clearly mark where the consequent clause begins, the use of both if and then can serve to emphasize the causal nature of the antecedent, or to make it seem like an if and only if rather than just an if-then.
For example:
If it rains, we will stay inside.
merely provides the plan of action in the case that it rains, whereas:
If it rains, then we will stay inside.
seems to suggest that the staying inside will only happen if it rains (note the emphasis on then, which would be stressed in speech and italicized in writing).
Best Answer
"...at least two or more..." is a frequently used pleonasm. So are "...at least two, or more..." and "..., at least, two or more ...". Two or more or at least two are the non-redundant equivalent sub-phrases.
However, a Google search for "at least two or more" can also yield this:
[The Photoshop Elements 13 Book for Digital Photographers, p. 118], emphasis mine
Why is "at least two or more" popular, despite its shortcomings? A few ideas.
Pleonasms have their uses. If it is vital that the text be understood correctly, even if only scanned in haste, then the formulation in question can be a preferred choice. Utility takes the better of logic.
Related to that, while "two or more" seems clear, people may have some trouble realizing what "at least two" means.
It is conceivable that the expected number or amount is much more than two; in that case, "at least two or more" can be uttered to mean "at least two, or much more". Ideally one would desire a phrasing that elides the numeral "two" altogether.
In spoken language, "at least two or more" can happen easily; one cannot take back what has already been said.
Interestingly, the collocation "at least two or three" is also quite popular, while not a bit less embarrassing from the strictly logical viewpoint. Of course, the collocation two or three can be conveniently used to convey a small but indefinite number.
On a related note, we find the following in the New Testament (Matthew 18:20): "For where two or three are gathered together in my name, there am I in the midst of them." Two or three. Does that cover four or more? Again, from a logical viewpoint, yes: if there are four, then there are also three, and two. Why then say "two or three", where merely "two" would have been sufficient? But then, can two actually gather?
While "at least two or three" gives about 32m hits, "at least three or four" gives about 1m. (I cannot resist quoting this:
[Mathematics placement test, University of Washington, Seattle]; emphasis mine)
Next, "at least six or seven" 174k; "at least eleven or twelve" 6.5k; etc. What is perhaps apparent here is that as the numbers zoom out to amounts indistinguishable by a quick glance (as one cannot, perhaps, tell apart a flock of twelve sheep from that of thirteen), people do not even bother to mention two separate values; it is less and less important how many items exactly are involved.
The trichotomy one / two / many (more than two) is/was ingrained in many European languages via their grammatical numbers (singular/dual/plural). This is how we count.