Every result (item/record) from trigger/iterator/list/search/match modules will output a bundle. This can result in multiple bundles, which then trigger multiple operations in future modules (one operation per bundle). To “combine” multiple bundles into a single variable, you’ll need to use an aggregator of some sort.
Aggregators are modules that accumulate multiple bundles into one single bundle. An example of a commonly-used aggregator module is the Array aggregator module. The next popular aggregator is the Text Aggregator which is very flexible and can apply to many use-cases like building of JSON, CSV, HTML.
You can find out more about the other types of aggregator modules here:
you can add the task name in a custom field and then use that to filter the tasks and see if it exists.
You can also have an internal clickup automation → when a tasks is created set that custom field to equal the task name.
Or like samliew suggested, aggregate the names coming form the List All Tasks module in an array and check if that array contains the name you are looking for.