Dear Cloudy, you can retake the five precepts as many times as wish, but when you take the precepts you are doing so with the idea of keeping them forever, so you should try as hard as possible to keep them. As a complete beginner, you should be trying to extend the span of time between successive failures
Karma comes in three flavours; good, bad and neutral, and it has three delivery systems; body, speech and mind.
If you kill an insect accidentally, it is neutral in terms of karma because you lacked the intention to kill. If a person with Tourettes swears at another person, they lack the intention to harm with words, and so this is also neutral. In everything 'it is the thought that counts', so a person who continually thinks of murdering someone is creating bad karma, even if they never go through with it. (Yes, Buddhism has Thought-Crime