It is very common to have different campaigns in your account and different ad groups with in your campaigns. It is always ideal to have different keywords in each of these ad groups. But some times you will end up with the same keywords (or similar keywords) in different ad groups.
First of all, this can happen accidentally especially if you have quite a few ad groups (or campaigns) running at the same time. It could also be that you are advertising for the same product or similar products which might all share some of the keywords.
Let’s see some of side effects of having the same keyword in different ad groups. Common sense and general assumption is that it will affect the campaign negatively. Does it really affect your campaign negatively?
keywords competing against each other
One of the most common questions is whether these keywords will compete against each other? The answer is it depends. It depends on the settings or configuration of the ad groups in your account.
If you have the same keyword in different ad groups, and if all of these have the same match type and is targeting the same audience, it is quite likely that they will compete against each other.
But one of the main reason, you have different ad groups is that they perform different functionality with in the campaign. They might be targeting different locations, running at different time periods, have different bid types, bid amounts or match types. Given all the different variations that you already can have between ad groups, it is quite unlikely that these keywords will actually compete against each other in a real scenario.
The probability of competition increases with the type of match type you use. In the case of broad match types, the is quite possible that both your keywords will match against a large set of keywords or queries. The probability of this happening decreases in the case of an exact match. Yet another reason, you should be using mostly exact and phrase matches exclusively.
similar keyword within the same ad group
It is quite possible that you have already similar keywords with in the same ad group that match the same search query. This is much more likely than the case of keywords across ad groups. In most cases, the match type and ad rank is considered to determine which keyword (and ad) wins. An exact keyword match will win against other types of matches and a higher quality score and ad rank will win if there are multiple keywords with the same match type.
In a real scenario, you will have many campaigns, ad groups and many keywords that are similar to each other. Many of these keywords will have different match types. Unless you use the exact match type exclusively, it is very likely that many of your keywords (with in the ad group and across ad group) match against the same search query already. Thus, it is competing against each other already.
So given all these variations, you should not worry too much about having the same keyword in different ad groups. You should remove them if you happen to find that they do not necessarily serve a function or business purpose. Other than that the impact of such keywords is usually very minimal.