Try restoring the default options in ‘Windows > Preferences > Java > Editor > Content Assist > Advanced‘
An example of the kind of data you see in this preference screen, however not necessarily what you currently have.

(From Vadim in this blog post ” Content Assist Duplicates in Eclipse (Mylyn)”:
if have duplicate Mylyn entries, uncheck the duplicate entries that do not contain “(Mylyn)” in their name)
The Eclipse help page defines the default list to restore:
Select the proposal kinds contained in the ‘default’ content assist list:
- Other Java Proposals,
- SWT Template Proposals,
- Template Proposals,
- Type Proposals