No, it's not illegal or unethical to buy more than one life insurance policy on the same insured.
When you apply for life insurance the application asks how much life insurance is already owned on the person you wish to insure.
The answer to this question helps the insurance company determine if the insured person is over-insured. Usually, there is no problem with a person being insured for up to 10-20 times his or her annual income.
And, there may be several life insurance policies on one particular person.
You may buy a term life policy and find out five years later you need additional coverage, and wish to purchase another term life plan to add more protection for your family.
Here's an article that explains how many life insurance policies can you have?