There are a few message delivery semantics on which message delivery guarantees (or lack of them) are based:

• At most one delivery: This means the message will be delivered zero or one times. It means the message may be lost. It's the cheapest way and is also known as fire and forget.
• At least one delivery: This means in multiple delivery attempts, there may be a chance that the message may be duplicated, but it will not be lost for sure.
• Exactly one delivery: This means that the message is only delivered once. It's neither getting lost or duplicated. 

Structured Streaming provides an end-to-end and exactly one message delivery guarantee based on the following features:

• Offset tracking in WAL
• State management (using the in-memory state and WAL)
• Fault-tolerant sources and sinks (using WAL)