I created a stored procedure, then I came back later to alter it and I
an error every time. Logging out completely and even rebooting
haven't fixed it, I have no idea what to do.

Here's the error & stored procedure:

ISC ERROR CODE:335544351

ISC ERROR MESSAGE:
unsuccessful metadata update
STORE RDB$PROCEDURE_PARAMETERS failed
attempt to store duplicate value (visible to active transactions) in
unique index "RDB$INDEX_18"

Statement: create procedure "sel_active_order_by_customer_id"
(
"user_id" varchar(50)
)
returns
(
"order_id" int,
"user_id" varchar(50),
"invoice_id" varchar(50),
"order_date" date,
"ship_date" date,
"order_status" varchar(50),
"payment_status" varchar(50),
"shipping_amt" decimal,
"tax_amt" decimal,
"order_amt" decimal,
"payment_message" varchar(100),
"avs_code" varchar(50),
"is_active" char(1)
)
as
declare variable "_order_id" int;
declare variable "_user_id" varchar(50);
declare variable "_invoice_id" varchar(50);
declare variable "_order_date" date;
declare variable "_ship_date" date;
declare variable "_order_status" varchar(50);
declare variable "_payment_status" varchar(50);
declare variable "_shipping_amt" decimal;
declare variable "_tax_amt" decimal;
declare variable "_order_amt" decimal;
declare variable "_payment_message" varchar(100);
declare variable "_avs_code" varchar(50);
declare variable "_is_active" char(1);
begin
for
select
"order_id",
"user_id",
"invoice_id",
"order_date",
"ship_date",
"order_status",
"payment_status",
"shipping_amt",
"tax_amt",
"order_amt",
"payment_message",
"avs_code",
"is_active"
from
"order"
where
"is_active" = 'Y'
and
"user_id" = :"user_id"
into
:"_order_id",
:"_user_id",
:"_invoice_id",
:"_order_date",
:"_ship_date",
:"_order_status",
:"_payment_status",
:"_shipping_amt",
:"_tax_amt",
:"_order_amt",
:"_payment_message",
:"_avs_code",
:"_is_active"
do
suspend;
end