Adding a value only once in a ManytoMany field across the table

I have a many-to-many field called paper in a model called Issues. Each Issues record shall have a list of papers. But the paper should be unique across the issue table.

In other words, a paper that is added once to Issues should not be able to be added in any other record of the Issues table.

How do i achieve it?

class Papers(models.Model):
    '''
        All the published papers
    '''

    title = models.CharField(max_length=300) # title

    def __str__(self):
        return self.title

class Issues(models.Model):
    '''
        All issues
    '''
    number = models.SmallIntegerField() # issue number
    paper = models.ManyToManyField('Papers')
    
    def __str__(self):
        return str(self.number)

You just need a foreign key relation for that use case

class Papers(models.Model):
        '''
            All the published papers
        '''
    
        title = models.CharField(max_length=300) # title
        issue = models.ForeignKey(Issues, on_delete=models.CASCADE)
    
        def __str__(self):
            return self.title
    
    
class Issues(models.Model):
        '''
            All issues
        '''
        number = models.SmallIntegerField() # issue number
        
        def __str__(self):
            return str(self.number)