In your campaign metrics reporting, you can view your data breakdown by keyword. To do this, click into a campaign, click the "Audience" tab, and select "Keywords".
Often when viewing this data, you’ll see that the campaign-level summary of all impressions, engagements, or spend is much lower than the sum of all the keywords listed below as line items. This is because there is a certain degree of overlap between keywords, which can occur when a single query or Tweet includes multiple targeted keywords.
For example, if you've targeted the keywords "sun" and "sunnyday", and a user has Tweeted "Can't wait for the sun! #Sunnyday", the ad impression served to that user would appear once next to "sun" and once next to "#sunnyday".
This is expected behavior, and you're only charged once even though it is reported twice (once in each row). You're only ever charged if the user makes the billable action of your campaign.