When a guardian uses two different names to submit a form for their child the lead will be created with two different guardians.
If you are certain these are the same people, navigate to the Family tab on the Lead Details Page.
Scroll down to the duplicate guardians and select the three action dots alongside their details as shown below. This will expose the option to merge.
In the merge modal search the guardian name, confirm their details and select merge guardians.
This is irreversible once actioned so be certain this is the correct guardian.
If you need to bring guardians into one family you need to follow the steps to merge families here.