Disclaimer: this bug has been filed in a semi-automated manner.
When emerging the package media-video/mjpegtools-2.0.0-r2, the following elog messages are displayed:
* Current versions of mjpegtools only support V4L1 which is not available
* for kernel versions 2.6.38 and above. V4L1 will be disabled.
Such messages should only be displayed when the package is a new install.
These situations can be checked in the ebuild as follows:
if ! has_version 'media-video/mjpegtools'; then
See the tracker bug 440214 for more details.
I think warnings and errors should be displayed all the time. If there is only an informational elog with information on how to configure or setup a package I agree that this message should be displayed only once.