Operating system and version: Ubuntu 18.04.2 LTS
Browser and version: Chrome Version 73.0.3683.86 (Official Build) (64-bit)
Version of BootstrapVue that you are using: 2.0.0-rc.15
Example: https://jsfiddle.net/vzotdshw/1/

When listening for the bv::collapse::state event, looks like it's being fired off 4 times (see console). This I believe is a bug as I would expect it to trigger only once.