Version 2.0.3 added a Setting (Notifications and Sounds — Repeat Notifications) that allows to select a repeat period or turn repeated notifications off altogether.

When you have unread personal messages, Android notifications will repeat every X hours (can be changed) until you open the message.

This only applies to messages in 1:1 chats. No groups, no service messages.

Reminder notifications are from 11:00 to 22:00 o'clock.