Linux Editor that can highlight all instances of a selected word

What version of Kate where you using, Kate has that exact feature as described here.

First install the addons plugin:

sudo apt-get install geany-plugin-addons

and then apply the answer of mr13.

To enable this feature in Geany do the following:

• Install the addons plugin: sudo apt-get install geany-plugin-addons
• In Geany, click on Tools -> Plugin Manager
• enable the plugin Addons (check in the column Active)
• configure the plugin by clicking on Preferences
• enable the option Mark all occurrences of a word when double-clicking it