<think>
Okay, I need to fix the install_order function according to the problem description. Let me start by understanding the requirements.

The current code uses a topological sort approach, but it's not handling the alphabetical ordering correctly. The problem says that when multiple packages are ready, the one with the name that comes first alphabetically should be chosen next. The current code is using a simple pop() from the ready list, which would take the last element, leading to the wrong order.

First, I need to modify the selection of the next package. Instead of popping the last element, I should find the lex smallest name in the ready list. So, in the loop, when selecting next_name, I need to sort the ready list and pick the first one.

But wait, the current code uses a list and pops from the end. To get the lex smallest, I should sort the ready list each time and take the first element. However, that would be O(n log n) each time, which might be inefficient for large data, but given the problem constraints, maybe it's acceptable.

Next, the validation part. The problem mentions three things to check:

1. No duplicate package names in the packages list. So, if a package is listed more than once, it's invalid.
2. Any requirement pair that references a package not in the packages list. So, if a required package is not in the packages list, that's invalid.
3. Cycles in the requirements. If there's a cycle, the current code would silently return a truncated order, but we need to reject it with an error.

So, I need to add these validations. Let's think about how to implement them.

First, check for duplicate packages. The packages list should have unique elements. So, I can check if the length of the set of packages is equal to the original length. If not, raise ValueError.

Second, check that all packages mentioned in the requires are present in the packages list. For each pair in requires, check if both pkg and needed are in packages. If not, raise an error.

Third, check for cycles. This can be done using a topological sort. If the number of nodes processed is less than the total number of packages, there's a cycle. So, during the topological sort, if the length of the order is less than len(packages), then there's a cycle.

But wait, the current code uses a Kahn's algorithm approach, which detects cycles by checking if the number of nodes processed is less than the total. So, in the current code, if there's a cycle, the order will not include all packages, and the function would return a partial list. But according to the problem, we need to reject such cases with an error.

So, in the code, after generating the order, we need to check if the length of the order is equal to the number of packages. If not, raise a ValueError indicating a cycle.

Putting this all together:

First, validate the packages list for duplicates. Then, validate that all required packages are present. Then, perform the topological sort, and check if the order includes all packages. If not, raise an error.

Now, modifying the code:

In the current code, the 'needs' and 'enables' are built. But for the validation, we need to check the requires. So, for each (pkg, needed) in requires, check if both are in packages. Also, check that packages have no duplicates.

So, in the code, first, check if len(packages) != len(set(packages)), then raise error.

Then, for each (pkg, needed) in requires, check if pkg is in packages and needed is in packages. If not, raise error