How does router advertisement in IPv6 avoid IP conflict? If a router advertisement sends the range of subnet, and the client can choose an IP from it, how is there no conflict with another client that has chosen the same address?

A host performs **Duplicate Address Detection (DAD)** before actually activating the given address on the interface

RFC 2462 has to say about this:

> 5.4. Duplicate Address Detection
>
> Duplicate Address Detection is performed on unicast addresses prior to assigning them to an interface whose DupAddrDetectTransmits
> variable is greater than zero. Duplicate Address Detection MUST take
> place on all unicast addresses, regardless of whether they are
> obtained through stateful, stateless or manual configuration, with
> the exception of the following cases: [...]

The original RFC was updated/modernized by RFC 4429 and further on by RFC 7527

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**EDIT** To nitpick a bit: the correct sequence of RFCs is more precisely:

RFC 1971, obsoleted by RFC 2462, obsoleted by RFC 4862, updated by RFC 7527