A very simple solution: Select the column (or columns) in which you want to enter the words. Click Home > Conditional Formatting > Cell Highlighting Rules > Duplicate Values > OK. As soon as you enter a word a second time, it will be marked by a light red background color.
Slightly more complicated: Select the column (or columns) in which you want to enter the words. As an example, let's assume it's column A, with A1 as active cell. Select Data > Data Validation. On the Settings tab, from the Allow list, choose Custom. Click in the Formula box. Enter the following formula - it refers to the selected column and the active cell:
Activate the Error Message tab. Enter an instructional error message if you want; it will be shown if you enter a duplicate word. Click OK.