In general, professional graphic designers try not to step on Intellectual Property Rights' toes, as a big part of our industry depends on them.*
What constitutes infringement, though? That is always a legal question, and not one with necessarily hard-and-fast rules. It will typically come down to the opinions of a legal team vs. the opinions of an opposing legal team and whatever the judge/jury decides.
If I worked for Aldi, this would be the lawyer's problem. Not mine.
If I am working for mom-and-pop store down the road, I'd want to sit down and explain my concerns about this potentially opening them up to a legal battle. If they still insist they I proceed, I'd either bow out of the project, or make sure my lawyer drafted up a contract that my client would sign releasing me from liability if they are ever sued.
*That said, I'll acknowledge the irony in that statement, as lots of designers have 'borrowed' fonts and copies of photoshop in their days...
I can definitely say that the first thing to do is to look at it in its context. See what the similarities are between all products of a similar or identical nature.
Once you know what the common, acceptable similarities are, you can identify what's unacceptably similar and could be infringing on your rights.
For my example, I'm going to imagine I designed the Kelloggs Coco Pops cereal box on the top left of the following image, and that the Choco Rice cereal sitting on it's right is a bit too similar for my liking.
This is an image of all of the competing products from the largest competitors:
To note all of the most immediate common similarities:
Each design has a bowl of cereal in it.
Most apart from the first three have left aligned and placed text.
Many of the designs include characters.
Half of them use varying shades of yellow backgrounds.
To note the questionable similarities:
Only two of them have a monkey character.
Three of them have a brown character with an element of blue.
Choco Rice shares an almost identical colour towards the bottom of the gradient, to the yellow in 'my' design.
Instead of using a blue hat, Choco Rice has incorporated an almost identical shade of blue on the bowl instead.
I would definitely take these specific points to a copyright lawyer/solicitor or IP consultant, to see if they are likely to succeed in legal action. I would focus on the copying of the character design, and seek to be compensated for piggybacking off of my designs recognition.
I'd also see if we could issue a warning to Asda for their questionable but less obvious similarities.
What is Property? Or, an Inquiry into the Principle of Right and of Government.
"Good Artists Copy; Great Artists Steal." - Steve Jobs
The same thing, clarified:
"One of the surest of tests is the way in which a poet borrows. Immature poets imitate; mature poets steal; bad poets deface what they take, and good poets make it into something better, or at least something different. The good poet welds his theft into a whole of feeling which is unique, utterly different from that from which it was torn; the bad poet throws it into something which has no cohesion. A good poet will usually borrow from authors remote in time, or alien in language, or diverse in interest."
TS Eliot, circa 1920
More succinctly, from 1967:
Igor Stravinsky said to me of his Three Songs by William Shakespeare, in which he epitomized his discovery of Webern’s music: “A good composer does not imitate; he steals.”
A better example:
“immature artists copy, great artists steal.” Knowing what and when to steal is very much a part of the designer’s self-education.
Faulkner, about 1974
One aspect of property is that it must be defined, measured and assigned.
If you've not done this with what you claim is yours, you've got no hope of doing anything other than paying lawyers to slowly, painfully, and very expensively demonstrate this reality to you. At your expense.
There are several huge reasons why this happens; 1 is because the client wants it like that. The way a client will look at this is "Well Coco Pops looks like this, and it sells, so a design like this will sell for us"
2nd is that particular design, tells a consumer a lot about the product on first glance, and sometimes that's all the time you have to sell the product. I see a yellow box of cereal with some brown in it, I'm thinking Coco Pops, sugar coated rice puffs with chocolate. Is that what my kids eat? Yeah, okay, get that. Done!
It's funny in the Movie Poster business, studios come up to you, show you 4 images off IMPAwards, tell you they want it to have X,Y and Z aspects, plus this cloud Steven in sales reckons looks good. "If you can make that happen that would be great." A reason they don't claim infringement on every other studio, is because they know everybody - they know that's what the client asked for. It's nothing personal, we just gotta get this stuff approved and paid. Which kinda throws a bit of light on that Spike Lee/Juan Garcia story... You got 4 design studios on the same job, one studio exec at the top, probably liked the coffin box idea and said "do this" but different, with the cloud Steven in sales reckons looks good!