Wix install sequence

I am trying to figure out the exact behavor of the install sequence in the setup, as defined in a wix .wxs file. The confusion comes from the presence of the InstallUISequence and the InstallExecuteSequence tags, leaving aside the fact that there are actually four tags related to the actions sequence (AdminUiSequence and AdminExecuteSequence are used in administrative installs). As it is said in the wix turorial, "InstallExecuteSequence is always consulted by the installer to determine the actions, InstallUISequence is only considered when the installer runs in full or reduced UI mode." So, if executing without user interface, the order is determined for the InstallExecuteSequence and no doubts remain. But, if executing in full or reduced UI mode, both sequences are consulted. So, what is the resulting order of execution? If an action is placed in both sequences, it is executed twice?

As for the executing order, what I suppose happens is that all actions in the InstallUISequence are run first (since they gather the information required for the installation) and then those from the InstallExecuteSequence. But it is possible that some actions need to be executed before any UI dialog, so then we should place them in both sequences.

If an action is placed in both sequences, they will be apparently executed twice. Al least one exception for this rule is the AppSearch action. In the schema reference it says: “AppSearch should be authored into the InstallUISequence table and InstallExecuteSequence table. The installer prevents The AppSearch action from running in the InstallExecuteSequence sequence if the action has already run in InstallUISequence sequence.”

For custom actions, the execute attribute can be used for preventing the double execution.