Visitor visa application while current visa is valid

[kirang]

Hello my parents are visiting me in July for a month and have a visa application appointment in India in June for a 6 months visa.
I want my parents to visit me again in December for a family function and live with me for 2-3 months. Since their visa will expire in December can they apply for another visa before travelling in December which will cover their stay for December to February ?

Basically what I want to know is can they apply for another visa while they have a valid visitor visa on their passport ?

I do not want to pay Rs. 28000 for 2 years visa as they may not visit me that many time and hence 2 6 months visa application seems most viable.