Nearly ALL babies go through a stage of calling all men “ daddy “ and all women “ mummy “. The same as they call all animals “ cat “ or doggy “ or their own pets name and all birds “ duck “ or “ quack quack “. And all vehicles “ car “ or “ bus “ .
It’s just a stage of language development when they think that word belongs to a whole category of things . Which makes perfect sense as all flowers are called “ flower”.
And no, it doesn’t usually cause great upset to the mum, because the baby is usually at least 10 months old by then and knows perfectly well who their mum is. It’s just a word error, it’s not mistaken identity.
Most mums are just excited that their baby is starting to talk and crawl.
So you can’t avoid it. All you can avoid is getting upset about it . Otherwise you need to avoid going to the supermarket or toddler group or anywhere your baby might see other women.
And don’t ever let them go to school, as they all call the infant teacher “mummy” at least a few times .
And don’t have more than one child yourself , in case YOU call them by their sibling’s name. Or the cat’s.
And let your mum chose her own name, the same as you get to choose your child’s name. This isn’t worth having a huge fight about - my advice is to choose your battles carefully and only choose those you can win.
This is a good lesson to learn in life , especially when you have a toddler.