Often times yes, however sometimes a contest's rules specifically state that only one entry per person/team is allowed. The best way to check on whether the contest you are entering allows for multiple submissions is to read the contest rules.
You can also determine if a contest allows multiple submissions by adding a project to the contest. If you add a project to the contest and see that there is an option to add another project, then the contest allows multiple submissions.
If a contest does not allow multiple submissions you will not see an option to add another project.