I fully grant that existence does indeed exist. However, that statement is rather ambiguous, as traditional philosophy makes a distinction between existence and Being – with the emphasis often placed on one over the other – because the idea of Being is not present in the empty tautology “existence exists.”
It is not to say that the concept of “existence” exists, as that would be obvious; but that a thing called existence exists that is not itself and can never be an existent. Because existents exist within existence. “Existence” is therefore synonymous with “universe,” as the sum-total of all that exists, that is, of all existents. I had to clarify this because the idea that “existence exists” is a mere tautology; it doesn’t prove that existents exist among other existents. It does not prove that multiple existents exist at all, because a universe or existence containing a single existent, even if it’s just a hydrogen atom, is all that is required for the concept of “existence” to be valid.
To bring traditional philosophy back into this, existence is Being, but we posit that existents possess the quality of Beingness (to break with traditional ontology). Because Being is traditionally the permanent in ontology. It’s seen as something that is not itself an existent but is an unchanging and fundamental quality that underlies all existents, such as the constants of physics. Individual existents change, but Being itself is permanent. And while existents themselves may exist in a flux, as Heraclites pointed out, they change in a regular, lawful, non-chaotic manner. So the monistic tendency of declaring that “existence exists” falls to the dualism of stating that, yes, existence exists, and not merely as an empty tautology, but that it also cannot exist without at least one existent contained within it (and not just subsumed beneath it as if they were mere concepts).
That singular existent, however, is not existence. Existence is something more; it contains the regulated lawfulness of the hydrogen atom in my example: the Beingness. Existence not only contains, it controls the existent, the hydrogen atom, as it changes, bringing to it a modicum of predictability. The term “beingness” is simply used to indicate that the existence of the existent is ongoing, permanent, that it may change its form but never cease to exist. “Beingness” only serves as a pointer to the permanence that exists in the existent. I am not therefore saying that “Beingness” is anything more than a concept. The existence is not Being itself, but we can posit that it possesses the attribute of Beingness without declaring that Beingness is itself an existent.