I am unclear on which equipment to use and not use. My class is rogue and my background is guide (which I found on this site). All the equipment that I get from the guide background is very similar to what comes in the explorer's pack. Do I keep the doubles or do I have to choose a different background perhaps?
This is up to you and your DM. There aren't rules to cover this problem because you won't face this problem when using published classes and backgrounds, because they don't have this problem.
The classes' starting equipment and backgrounds' starting equipment in the PHB don't suffer this problem. Generally speaking, you get some weaponry, armor, a pack, and things related to your class features1 from your class' starting equipment. From your background you get clothing, a few knick-knacks,2 and some money. The biggest overlap I can find is if you're a Wizard with the Sage background: you might end up with two bottles of ink rather than one.
(Compare with the similar situation with proficiencies: you can absolutely "get" the same skill or tool proficiency twice through class and background, so there's a rule to cover this on PHB p.125.)
1 - things like a spellbook, holy symbol, musical instrument, thieves' tools, &c.
2 - things like a trophy from an animal you killed, a rabbit's foot, militaria insignia; avoiding the term "trinket" because they don't overlap much with the trinkets table on pp.160-161
You get all of the equipment from both sources. The intent is that your chosen race, class, and background all combine to form the core of your character. See page 14 under "Choose Equipment":
Your class and background determine your character's starting equipment, including weapons, armor, and other adventuring gear.
If you're concerned about duplicates, you have two options. First, you could just have backup items. Second, you could talk to your DM about using the purchase alternative rules in the second paragraph in the above referenced section.
As for changing your background, I'd advise against it from a roleplaying perspective. Backgrounds are there to define who you are, where you came from, and how you see the world. The point is not the stuff you got from doing that.