I don't fully understand. The highlighted line in the screenshot is a duplicate of the line a bit higher. The gray background in the other line is just a feauture of the editor to show you where the currently selected phrase occurs in this file again.
What did you do to duplicate the identifier? Normally it is quite difficult to achieve this...
Every identifier must be unique. Add a number, or a descriptive suffix/prefix to the base name. Enter the new name in the object inspector, row "Name".
Not sure, but once a duplicate component identifier is in the file the IDE may have huge difficulties to rename it. Close the IDE, make a backup copy of your project, open the pas file in an external editor, rename the identifiers so that they are different. Then open the lfm file (with the same base name) in the editor and find the two editors and rename them here again. Then load the project into Lazarus and try to compile it. When there are still references to the old names somewhere in your source code, it will tell you where they are; change the names accordingly.