Euphemism is the substitution of something that might be offensive or hurtful with something more innocuous. Repetition is the purposeful re-use of a word, phrase, image or sound, and is fundamental to poetry. In high school, you may have had your English teacher circle repeated words and say that is was bad or ineffective. Unintentional repetition can be, but do not think any repetition is a sin. It can be salvation.
Symbol is an image that represents or stands for something else.
Flags, for instance, represent countries. The Statue of Liberty represents freedom. In your own work, you may create symbols that are true to your piece alone. In The Great Gatsby, the green light at the end of the dock becomes a symbol for hope. In The Big Sleep, the image of a knight comes to represent Marlowe.